Initial Approach to the National Application

In determining an initial approach to building a national application, five considerations were apparent from the start:

The USGS operates offices in most States, and the State offices (or the regional centers encompassing those offices) often form cooperative agreements with other Federal, State, and local agencies to share costs for USGS scientists to perform work of mutual interest. In these cooperative agreements, the USGS can provide no more than half of the cost for the work, and often the USGS contribution to costs is substantially less. From the start of the national StreamStats effort, it was determined that federally allocated funding would be used to support the national development team and that USGS State offices would need to form cooperative agreements with other agencies to fund the work needed to implement StreamStats for individual States. Cooperative funding would allow USGS scientists in the State offices to prepare the large GIS datasets required for implementing StreamStats and to work with the national development team on implementation. This approach lessened the cost to the USGS, assured the interest of the State cooperating agencies, and allowed for innovative customization to meet the unique needs of a cooperator. A drawback with the State-based shared-cost approach is that work to implement StreamStats for a given State generally could not begin until a cooperator could be found to help pay for the cost of implementation (as of 2023, only one State did not have a StreamStats application or a cooperator agreement in place with a water science center to develop an application). Another drawback with the State-based approach is that water does not recognize State boundaries. Implementation of StreamStats using a watershed-based approach would have made more sense hydrologically, but taking that approach would have required the difficult task of getting agencies from multiple States to cooperate in order to implement StreamStats for a watershed. Also, existing regression equations for estimating streamflow statistics were almost all developed on a statewide basis, so implementing StreamStats on a watershed basis would have required resolving how to provide estimates for user-selected sites when the drainage areas for those sites fell within multiple States or developing new watershed-based regression equations.

The national version of StreamStats was designed to have a single user interface (front end), with each State set up as a separate application running in the background (back end). This approach eliminated the need to design a separate front end for each State and allowed users to have a single point of access nationally. A watershed approach was used in a few cases where there were specific needs for such an application, as described in the “River Basin Applications and Custom Functionality” section.

Source Data for Delineations and Computing Basin Characteristics

Delineations of basins in StreamStats rely on the horizontal synchronicity of three source datasets: a DEM grid, a vector stream network, and a vector watershed boundary dataset. The accuracy of the basin delineations is highly dependent on the resolution and precision of these datasets, which generally have steadily increased over time (fig. 14). Over the years, the StreamStats development team has coordinated closely with the teams that developed the national core datasets described in the following paragraphs to assure that the timing of data delivery and the structure and quality of the data meet the needs of StreamStats and other interested parties.

Elevation data.—The NED was available nationally at the start of the national StreamStats effort at a 30-m grid-cell spacing, with some areas available at a higher resolution. Most States initially were implemented by using the 30-m data from the NED. For many of these States, the 30-m NED data were resampled to 10-m resolution during the data preparation process. A few States were implemented by using locally available data at a resolution that was higher than the NED.

The elevation data currently used to implement and update StreamStats generally are taken from the USGS 3D Elevation Program (3DEP; U.S. Geological Survey, 2020a), which is the successor to the NED. The scales and sources of elevation data available from the 3DEP vary geographically. Much of the data now available from 3DEP were derived from light detection and ranging (lidar) and are much more precise than the older data from the NED. StreamStats now uses data at 10-m resolution for most implementations. Beginning with North Carolina in 2007, some States have chosen to implement or update StreamStats using higher resolution data, and that trend is expected to continue. StreamStats for 17 counties in western North Carolina was implemented using stream vector data at 1:4,800 scale and DEM data at 20-foot scale generated from lidar (Weaver and others, 2012). Since then, the St. Louis area in Missouri (Southard and others, 2020) was implemented and updates to South Carolina (Feaster and others, 2018) and New Jersey (Watson, 2022) were done using high resolution lidar-based data. The lidar-derived elevation data provide improved accuracy of delineations in relatively flat terrain and for small basins and allow for the potential development of tools for determining channel properties and providing base-level engineering mapping.

Hydrography.— The National Hydrography Dataset (NHD) provides a seamless digital representation of the surface water of the United States (Simley, 2018; U.S. Geological Survey, 2020b). This dataset contains a vector stream network that allows a user to digitally navigate to track water upstream or downstream from any point on the network, a process that is also referred to as tracing. The NHD also allows linking of features such as rivers, streams, water bodies, canals, streamgages, dams, water withdrawals, and point discharges as “events” on the network, with associated attributes. The combination of user-tracing capability and linked events in the database helps with the analysis of cause-and-effect relations, such as whether (and how far) a streamgage is upstream or downstream from a dam that affects the flow of water in a stream, or whether there are any industrial or municipal wastewater discharges upstream from a water-supply intake. This user-tracing capability also allows tracking of an actual or potential contaminant spill through the stream network to help users understand the timing of the movement of the spill and what downstream activities may be affected by it.

The NHD is available nationally as medium-resolution and high-resolution products. The medium-resolution NHD was available at 1:100,000 scale when the national StreamStats effort began. This dataset was developed through a collaboration between the USGS and the U.S. Environmental Protection Agency (EPA) beginning in the late 1990s. A high-resolution, 1:24,000-scale version of the NHD, built in cooperation with the USGS, EPA, and many additional Federal, State, and local agencies, was released nationally in 2007 and, since then, has been the primary hydrography dataset used by StreamStats. The high-resolution NHD is essentially a compilation and sophistication of the original DLG. More recently, NHD with scales of at least 1:5,000 (termed “local-scale NHD”) has been or is being developed in many areas, and some of these local-scale data have been used to implement or update StreamStats.

A large part of the effort needed to implement StreamStats for a State involves quality assurance, editing, and cleanup of the NHD. This work includes making sure that NHD stream reaches have flow directions pointing in the correct direction, removing isolated streams that are not part of the network, and removing braids and canals to create a dendritic network so that flow through the network is confined to a single path (fig. 15). In addition, streamlines are broken at locations of headwater wetlands with multiple streams flowing out of them so that flow from the wetlands would be in only one direction. The final, cleaned-up version of the NHD resulting from this process generally was used in the burning process of the New England Method to generate elevation datasets that were synchronized with the NHD.

Watershed boundaries.—A nationally available digital dataset of watershed boundaries also was not available at an appropriate scale when the national StreamStats effort began. For States whose applications were implemented early, a digital dataset of previously delineated basin boundaries, usually based on streamgage locations, was provided by the local USGS offices or their partner State agencies, then used in the New England Method walling process to prepare the StreamStats data.

The Watershed Boundary Dataset (WBD) is a hierarchical system of watershed boundaries that were mapped based on surface features at a scale of 1:24,000, by using nationally consistent standards (U.S. Geological Survey and U.S. Department of Agriculture, Natural Resources Conservation Service, 2013), except a scale of 1:25,000 was used in the Caribbean and much of Massachusetts and a scale of 1:63,360 was used in Alaska. The WBD was developed by the National Resources Conservation Service (NRCS) of the U.S. Department of Agriculture and by the USGS, under the coordination of the Advisory Committee on Water Information’s Subcommittee on Spatial Water Data. The WBD divides the Nation into 22 regions that are each assigned a name and 2-digit code. Within each region are as many as seven additional nested levels of subdivision, each with its own assigned name and 2-digit code. Drainage-area sizes at a particular level within the hierarchy vary according to the dictates of the hydrology. The WBD is complete nationally at the 12-digit, subwatershed level. The subwatersheds typically range in drainage-area size from about 15 to 60 square miles (mi²), with an average area of 36 mi².

The WBD began to be available for individual States in the early 2000s and was typically used in the walling process for StreamStats for States where it was available at the time of implementation. The WBD included basin boundaries for most streamgages with long-term continuous data, but it often did not include basin boundaries for streamgages with shorter records. Consequently, the WBD boundaries were sometimes augmented with boundaries for short-term streamgages that were developed by the individual USGS State offices.

NHDPlus.—The NHDPlus is a set of integrated vector and raster geospatial datasets that provide a national model of how water flows across the landscape. NHDPlus was developed by the EPA, with assistance from the USGS. NHDPlus integrates snapshots (copies of datasets taken at a specific point in time) of the 1 arc-second (approximately 30 m) NED, 1:100,000-scale NHD, and the 1:24,000-scale WBD, and it includes several derivative datasets and value-added attributes that help with hydrologic analysis (U.S. Environmental Protection Agency, 2020). The NHDPlus dataset provides all the data (hydrography, elevation, watershed boundaries, and derivatives) needed to implement drainage-basin delineations in StreamStats. NHDPlus Version 1 was released in 2006 and used to implement StreamStats for California, Idaho, Oregon, Washington, and Wisconsin (Idaho was updated in 2009 using high-resolution data). NHDPlus version 2 was released in 2012 and included substantial improvements over version 1, including improved base datasets, improved processing procedures, and additional attributes. Version 2 was used to implement StreamStats for Kentucky, Montana, and South Carolina.

In 2016, the USGS began an effort to develop a new, high-resolution version of NHDPlus, named NHDPlus HR, and released an initial version in 2022 (U.S. Geological Survey, 2023a). NHDPlus HR was built by using 1/3 arc-second (10-m ground spacing) elevation data from the 3DEP program, along with 1:24,000-scale or better NHD data and 1:24,000-scale WBD data, and it includes many new attributes to provide enhanced functionality. The StreamStats application for Maine was implemented in 2015 using an initial version of NHDPlus HR data and has been the only State to use this dataset. The Maine application was updated in 2023 to use watershed boundaries derived from high-resolution lidar DEMs.

Basin characteristics.—Approximately 275 unique basin characteristics appear in USGS regression equations. The basin characteristics and the source data used to compute them vary widely among the States for which StreamStats has been implemented. Typically, the basin characteristics—such as drainage area, stream slope, and mean basin elevation—that can be computed for a State include all of those used as explanatory variables in the regression equations developed for the State; often, other basin characteristics can be computed, adding value to the application and extending the usefulness to many other scientific or management questions. StreamStats provides a web page at https://streamstats.usgs.gov/information-portal/ that contains links to lists of streamflow statistics, basin characteristics, report citations, and descriptions and metadata of the geospatial data used to implement the application for each State. The individual State pages can be accessed by selecting the State name from that web page.

In 2022, the USGS began implementation of the 3D National Hydrography Program, (3DHP), which is planned to generate new hydrography data for the Nation over 9 years to provide better support for hydrologic modeling and accounting (U.S. Geological Survey, 2023b). The hydrography is being derived from 3DEP DEMs extracted from 1-m lidar, except for Alaska, which is using 5-m DEMs from interferometric synthetic aperture radar. The 3DHP inherits key attributes of the NHD, WBD, and NHDPlus HR and is much more closely integrated with topography derived from 3DEP than those legacy datasets, which the 3DHP will replace. It is anticipated that the new 3DHP data will be used in StreamStats as they become available.

Database Programming Assistance

When Massachusetts StreamStats was implemented, the regression equations used for estimating the streamflow statistics produced by the application were hard coded within the computer programming. A new, national StreamStats application would require a database of the information needed to solve the regression equations for estimating streamflow statistics for each State. Also, the database that held all the previously published statistics for streamgages in Massachusetts would need to be modified and expanded to hold similar information for streamgages nationally.

In 1994, the USGS began distributing a desktop program, named the National Flood Frequency (NFF) program, that solved regression equations for estimating peak-streamflow statistics for ungaged sites. NFF users needed to download and install the program and an associated database before running the program. NFF users would then need to specify (1) a State, (2) a region within the State, and (3) the values of the basin characteristics used as explanatory variables in the regression equations to receive estimates of the streamflow statistics for a site of interest. The NFF program was modified in 2004 to allow estimating other types of streamflow statistics besides those for peak flow, and it was renamed the National Streamflow Statistics (NSS) program (Ries, 2007; U.S. Geological Survey 2019). The NSS program does not include a means for determining the values of the basin characteristics for a site, so users need to determine them by other means. The national StreamStats application took this NSS functionality and incorporated it into a map-based user interface to fully automate the process of selecting a site, computing the basin characteristics, and solving the regression equations.

The NSS program was written in Microsoft Visual Basic, and the database was developed in Microsoft Access by Aqua Terra (now a part of RESPEC) under contract to the USGS. Rather than the StreamStats team attempting to develop its own databases, it was decided that it would be more efficient to have the contractor modify the NSS program and database so the NSS program could be run as a background process by StreamStats. Aqua Terra then modified the NSS program so that it could solve all types of regression equations, such as equations for mean flows and low flows—thus the name change from NFF to NSS. Aqua Terra also combined their Access database for solving regression equations with an Access database that contained previously published streamflow statistics for Massachusetts and made further modifications needed for use in the national StreamStats program. As of 2023, this combined database contained the data needed to solve nearly 7,600 regression equations nationally, and about 2.35 million streamflow statistics for nearly 36,500 USGS streamgages.

The NSS program has many additional capabilities besides solving regression equations to obtain estimates of streamflow statistics for sites of interest, such as estimating weighted streamflow statistics for streamgages and ungaged sites and providing flood frequency and flood hydrograph plots (U.S. Geological Survey, 2019). The USGS has developed a web-based version of NSS that is available at https://streamstats.usgs.gov/nss/. The current (2023) web-based version provides only the functionality for estimating streamflow statistics for ungaged sites because functionality for estimating weighted streamflow statistics for streamgages and ungaged sites is undergoing testing.

GIS Programming Assistance

The StreamStats development team initially contracted in 2002 with a consulting firm to help build a new, web-based user interface that could be implemented nationally. However, this initial contracting effort was not successful, and another approach was needed. Thus, the development team entered into a cooperative research and development agreement (CRADA) with Esri (formerly Environmental Systems Research Institute or ESRI) in 2003 to determine a programming approach for the new national application. At about this same time, Esri was working with the University of Texas at Austin to develop the ArcHydro data model and toolset (Esri Water Resources Team, 2014). The ArcHydro toolset is a no-cost add-on to Esri’s ArcMap software and is used to process DEMs, define streams and catchments, delineate watersheds, and perform other hydrologic analyses. The StreamStats team and Esri decided to take advantage of some of the tools that were already built into the ArcHydro toolset, and linking ArcMap to ArcIMS to enable web functionality was more efficient than developing new programming from scratch. The New England Method of basin delineation, which minimized reliance on the raw DEM for basin delineations, was already included in ArcHydro. Adoption of the use of the ArcHydro toolset and ArcIMS for implementing StreamStats minimized the need for command-line programming for data preparation, making it easier and more efficient for the GIS specialists in local USGS offices to participate in the StreamStats implementation process and gain expertise that would be applicable to many other projects. Additionally, the knowledge gained from using ArcIMS through the CRADA with Esri enabled the StreamStats team to provide support for personnel from local USGS offices who were learning the then-emerging technology for web-enabled mapping applications. Under the CRADA, several new tools were added to ArcHydro for use in StreamStats, and those tools eventually were released in the public version of ArcHydro (current and previous versions and documentation are available at https://www.esri.com/en-us/industries/water-resources/arc-hydro).

The needs for increased stability and speed of delineations were major issues that had to be resolved before StreamStats could be made available nationally. The Massachusetts application had to be closely monitored and reset often to remain online. This level of monitoring would not be possible with a national application including many States; thus, stability was a critical need. The delineation speed of the Massachusetts application was acceptable for that State, but implementation for most other States would require processing much larger GIS datasets than those for Massachusetts. As a result, a more efficient means of on-demand processing of the data was needed to deliver results to users in a timely manner. As GIS datasets have gotten more detailed over time, this need for increased processing speed continues to be a challenge.

The Esri ArcHydro team determined an approach for increasing the basin delineation speed that involved splitting the geospatial data for each area to be included in StreamStats (usually States) by 8-digit hydrologic unit codes (HUCs) (U.S. Geological Survey, 2021) that average about 1,540 mi² nationally. Any delineation that required a drainage area larger than an 8-digit HUC used a two-tiered functionality designed by Esri: the 8-digit HUC that contained the user-selected site of interest was designated the “local” unit, and upstream HUCs were designated “global” units (fig. 16). Processing needed to define the basin boundary within the local unit for the user-selected site was done interactively. Each local HUC has an associated upstream global HUC except for headwaters HUCs. Drainage areas and other basin attributes for the upstream global HUCs were precomputed so that the total drainage area and other basin characteristics for the site could be determined quickly through mathematical operations (such as summing and averaging) on the values from the local and global units.

Since the earliest days of national implementation, the three base datasets (elevation, hydrography, and basin boundaries) were preprocessed to generate the derivative layers needed for basin delineations. As was done for Massachusetts StreamStats, preprocessing began by imposing the vector streams on the raster elevation data to generate flow-direction and flow-accumulation grids, in which the resulting elevation data were forced to agree with the original vector streams. An optimal threshold number of cells to indicate the upstream end of flowing streams was determined experimentally from the flow-accumulation grid, and the resulting threshold was used to generate a stream-definition grid. From these generated grids, vector stream reach and catchment layers were developed, in which stream reaches generally are lengths of stream between confluences and catchments are the drainage areas that contribute to individual stream reaches. The reach layer was given several attributes to allow determination of properties such as length, slope, and flow direction. The catchments also were given attributes to allow determination of such properties as area and flow direction. This synchronization of the raster and vector datasets for delineations enhanced the capabilities of both datasets, which later led to the development of virtual stream network navigation capabilities for users in StreamStats.

With the geospatial data preprocessed, StreamStats was able to interactively determine the drainage boundary for a new user-selected site within a catchment up to the points at which the new boundary intersected with the existing catchment boundary. StreamStats could then use the catchment attributes to identify any upstream catchments and determine the total basin area for the site. This approach limited the most intensive computer processing to the definition of the new boundary within the initial catchment. However, additional processing of the upstream catchments was still required to determine any needed basin characteristics for the selected site.

The Esri ArcHydro team devised a major innovation by designing an additional derivative-vector layer, called the adjoint catchment layer, that allowed optimization of processing speed within the local HUCs. Preprocessing to generate the adjoint catchment layer used a nesting technique such as the one used to determine global HUCs at the 8-digit HUC level. The adjoint catchment for the catchment of a user-selected site is a single polygon that includes all catchments that are upstream from the initial catchment and has the basin characteristics precomputed as attributes. The addition of this layer substantially reduced the amount of drainage area that needed to be computed on the fly. All six derivative layers (three raster and three vector) work behind the scenes in StreamStats.

Esri also implemented methods to compute a handful of complex basin characteristics, the most important of which was stream slope, which is used in many USGS regression equations. The basin characteristics used in the regression equations varied widely among the States, so the programming needed to compute the characteristics was customized for each implementation of StreamStats by the State offices using ArcHydro XML, an extensible markup language that is customized for use in ArcHydro. More information on ArcHydro, including documentation and installation instructions, can be found at https://www.esri.com/en-us/industries/water-resources/arc-hydro.

The final result of the CRADA with Esri came in January 2005, when the first State, Idaho, was released in national StreamStats, version 1. The new application allowed for a separate map projection for each State and variation of scales in the GIS data used for implementation. These features were important because State cooperators generally wanted to see their States presented in the StreamStats user interface in the projection that they were accustomed to, and the scales of the GIS data needed to implement StreamStats varied widely among the States at that time. The CRADA also resulted in the ability for users to edit basin boundaries and download boundaries, basin characteristics, and flow estimates in shapefiles. Esri also was contracted to assist with resolving many other issues with StreamStats after the initial CRADA expired.

Other Geospatial Considerations

Implementation of StreamStats for each State has required developing map layers for hydrologic regions that correspond to areas within that State where the regression equations for estimating flow statistics for ungaged sites are applicable. Most States have multiple hydrologic regions within the State for a given type of flow statistic, such as peak flows, and many States have regression equations for multiple types of flow statistics, each of which requires a separate map layer of the hydrologic regions for that type of flow statistic. Hydrologic regions within a State usually are defined based on differences in physiography, climate, or both. In most cases, external boundaries of the hydrologic regions correspond with the State borders so that StreamStats users are unable to get estimates of streamflow statistics using the equations for one State when a selected site is outside of that State. In some cases, however, the hydrologic regions extend into adjacent States, such as the Yellowstone River Basin in northwestern Wyoming, where basin delineations, basin characteristics, and flow estimates can be obtained using the Montana application, and the District of Columbia, which has no separate regression equations and has been incorporated into the Maryland application.

Map layers that were termed “hard” and “soft” exclusion zones were set up for many States. When StreamStats was implemented for a particular State, it was necessary to process geospatial data from parts of adjacent States with drainage areas that drained into the State of implementation. Hard exclusion zones were used to disallow delineations for selected sites in the adjacent States when using StreamStats for the State of implementation. Hard exclusion zones also were often established along lengths of major rivers to disallow estimation of flow statistics if the drainage areas for sites along the excluded reaches were much larger than the maximum drainage area for the streamgages used to develop the applicable regression equations, or if flows along those reaches were manipulated to the extent that estimates from regression equations would not reflect actual conditions. Soft exclusion zones were established in some areas where regression equations do not apply, such as southeastern Massachusetts (including Cape Cod), where, because of sandy soils, drainage divides defined by surface topography do not agree with divides determined from mapping of groundwater levels. Users can receive the typical results in soft exclusion zones, but with warning messages.

The approach of using flow-accumulation and flow-direction grids as the basis for determining basin boundaries tended to be inaccurate in flat areas, where elevations in the non-lidar-derived source DEM were nearly all the same. Other types of terrain also presented unique challenges, including along coastlines, in areas with closed drainage and areas with karst topography that formed sinks, in some urban areas, and in inland wetlands and water bodies. Special data-processing steps that had to be taken for these areas are explained in the following paragraphs.

Coastlines.—Delineations of basin boundaries at or near the land/sea interface often were incorrect because of the flat terrain along many coastlines. In most States with this issue, the land/sea interface was addressed by (1) redefining the geographic extent of the local HUC to include an adjacent chunk of offshore area; (2) extracting the coastline features from the NHD and using these features to manipulate the DEM data so that all elevation values on the land side of the coastline with values of 0 or less were assigned a small positive value (in most cases, 1 centimeter) and all sea-side values were assigned 0; and (3) extending the dendritic network out and through the sea portion of the redefined HUC. After these adjustments, the standard ANUDEM and AGREE tools could be used for defining basin boundaries.

Sinks.—Types of terrain that were treated uniformly as sinks included karst areas, prairie potholes (mostly in the upper Midwest), alluvial fans where flows from upstream are absorbed into the sediments, and closed basins in much of the West. Points were manually placed at sink locations during the data preprocessing, rasterized, and assigned a special value in the flow-direction grids. These values were then treated as outlet pour points so that upstream flow would be considered as being from within the sink's watershed. Delineations at a sink point will result in a sink watershed. When it is known where the flow eventually ends up downstream (for example, flow reemerging to the surface through karst or storm drains), these areas can be included in the larger stream network.

Urban storm-drain networks.—Mapped streamlines in urban areas often are discontinuous, with breaks in the streamlines occurring when the streams are diverted into underground channels to become part of the storm-drain network. As a result, drainage-area delineations in urban areas often are erroneous. Recently, the approach described above for sinks was applied to some urban storm-drain networks where the storm drains were treated as sinks, and delineations at storm drain locations captured the drainage areas that contributed to flow into them through open channels and storm-grate catchments. This approach was first applied to the St. Louis metropolitan area of Missouri (Southard and others, 2020), where storm sewer pipes 8 inches or greater in diameter were digitized and connected in a GIS to the storm drains and surface stream networks to help with accurate computation of drainage areas anywhere along the storm sewer and stream networks (fig. 17). This approach of treating storm drains like sinks, which was developed with assistance from Esri, has since been applied to watersheds within Washington, D.C., and in the Mystic River Basin in Massachusetts. High-quality elevation data, such as lidar, and a comprehensive geodatabase of the storm sewer system are needed for this type of processing.

Inland water bodies and wetlands.—Because inland water bodies and wetlands are flat areas on the landscape, a banding effect often can be seen in stream grids generated from the DEM. A bathymetric-gradient process was applied to overcome this banding effect by using the vector stream network to define more realistic stream grids within the water bodies and wetlands. This process creates slopes in the DEM from the edges of the water bodies and wetlands inward toward the elevation grid cells that correspond with the enclosed vector streams (fig. 18). The effect is to create a concave bowl within the water body or wetland, with the stream at the center of the bowl. The process then generates the elevation derivatives from the modified DEM. This process also has the added effect of enforcing delineations at the outlets of water bodies to honor the shoreline rather than cutting though a portion of it.

Coordination of Activities

The StreamStats development team interacts with scientists in the USGS State offices throughout the process of implementing StreamStats for individual States. This process starts with either the USGS State office approaching State agencies or other potential cooperators about the possibility of cooperating on a project to implement StreamStats, or the USGS being approached by other agencies to implement StreamStats. In either case, scientists from the USGS State offices develop a project proposal describing the purpose and scope of the proposed project, the approach to be taken, the costs, the personnel to work on the project, and the timeline for the work to be done. StreamStats implementation proposals often include the development of new regression equations for estimating streamflow statistics in addition to implementing StreamStats. The project proposal is then presented to the potential cooperating agencies. Projects typically cannot begin until one or more agencies sign an agreement to fund at least half of the cost of the project. In projects that have been completed already, cooperators usually have paid substantially more than half of the cost, with the USGS funding the remainder.

Costs for StreamStats projects have varied widely among the States, depending on the size of the State, which dictates the amount of GIS data to be processed; types and numbers of regression equations to be implemented and basin characteristics used in the equations; whether new equations were developed as part of the project; and whether new functionality or other work was included in the projects. Scientists in State USGS offices are required to consult the StreamStats development team in the process of developing proposals to assure that project costs and proposed timing will be adequate to meet objectives. The StreamStats development team is required to provide reviews of all completed proposals before any agreements can be signed, which also assures that the team is aware of the scope of the work to be done and the timeline for when it is needed.

StreamStats development team GIS experts usually trained GIS specialists in the USGS State offices to generate the geospatial datasets needed to implement StreamStats for their States, including (1) DEM derivative grids, (2) updated and edited NHD streams, (3) datasets needed to compute basin characteristics, (4) hydrologic region boundaries, (5) any special display map layers, (6) streamgage locations, and (7) exclusion areas. Knowledge gained through StreamStats data preparation has often led to other program-development opportunities in the USGS offices. In some cases, a USGS State office did not have a GIS specialist with the needed skills, and the work had to be given to a GIS specialist from another USGS office or a specialist on the national development team. Completion of the data-preparation tasks required very close communication between the State and development team GIS specialists, and, in a few cases, personnel from cooperating agencies and other organizations assisted with the GIS data preparation.

Quality assurance of the basin-boundary and stream-network datasets usually was a large component of the data-preparation process. Typically, a GIS specialist from a USGS State office prepared sample data for a part of the area for which StreamStats was to be implemented and provided it to the development team GIS specialists for review before proceeding with data development for the rest of the area. The State GIS specialists often forwarded corrections to the NHD and WBD data for incorporation into those datasets. A combination of DEMs with 10-m grid spacing, 1:24,000-scale streams, and 1:24,000 basin boundaries was used to develop the derivative grids for most States. Some States used different combinations of data scales, depending on data availability and financial resources at the time of implementation. StreamStats was initially implemented for California, Oregon, Washington, and Idaho by using NHDPlus version 1 data, developed from 30-m DEMs, 1:100,000-scale streams, and 1:24,000-scale basin boundaries (U.S. Environmental Protection Agency, 2020). Idaho was updated in 2009 to use the typical combination of higher resolution data.

After the GIS specialists in State USGS offices delivered their complete data to the national development team, the team would set up a test StreamStats website to enable testing of basin delineations, computation of the basin characteristics, and determining estimates of streamflow statistics for user-selected sites. The locations of a group of streamgages that had been used to develop the regression equations would be submitted in a batch process to the test site. The output from the batch process would be compared to the previously published basin characteristics and flow statistics for the streamgages, and the accuracy of the StreamStats results would be evaluated and documented.

Problems with StreamStats test results were often found during the initial testing. Efforts required to fix the problems varied widely among the States for which StreamStats was implemented. These efforts ranged from fixing a typographic error in a regression equation to having to develop entirely new GIS datasets or regression equations from scratch. In a few cases where the regression equations were developed by using basin characteristics that had not been computed with a GIS, the basin characteristics could not be reproduced adequately, so the equations could not be implemented. After all the obvious problems were eliminated, the cooperating agencies were invited to thoroughly test their individual State applications. Applications were released to the public when the USGS and the cooperating agencies agreed that StreamStats was providing accurate information. The StreamStats development team, scientists from the USGS State offices, and cooperating agencies worked together to write an application information page that is provided for each State in StreamStats.

The StreamStats development team has met regularly for several years with other USGS internal and external entities to coordinate activities and exchange information. Regular meetings are held with the

River Basin Applications and Custom Functionality

A map showing the status of where StreamStats has been implemented or is undergoing implementation, including the locations of the river basin applications, can be found by clicking on the About button at the top right of the StreamStats user interface. StreamStats applications have been set up for the Connecticut River Basin (in parts of Connecticut, Massachusetts, New Hampshire, and Vermont), Delaware River Basin (in parts of Delaware, Maryland, New Jersey, New York, and Pennsylvania), and the Lake of the Woods–Rainy River Basin (in parts of Minnesota and the Canadian Provinces of Manitoba and Ontario). The river basin applications were set up to address needs that were specific to those areas:

Cooperating agencies for several States requested that custom functionality be included as part of implementing StreamStats. In addition, implementation of StreamStats for a State sometimes spawned separate applications that relied on StreamStats web services for some functionality. Custom functionality developed for some States has occasionally been adopted for use by other States. States with unique functionality include the following:

Many other States have also requested the ability to compute unique basin characteristics, or to compute certain basin characteristics that are computed for other States, but in a way that is unique for their State. For brevity, these customizations are not listed above. Information about custom functionality or a separate application for a State can be found, along with links to additional information, by clicking on the About button on the StreamStats user interface after first selecting the State or region of interest.

Advisory Committee

The size and scope of the StreamStats effort required decisions on technical and policy issues that had to be accepted throughout many USGS offices and by the cooperating agencies. As a result, the StreamStats Advisory Committee (SSAC) was formed in 2004 to evaluate these technical and policy issues and to recommend technical approaches, development priorities, and policies needed to successfully implement StreamStats nationally to the development team and USGS management. Initially, the SSAC included 10 members who were selected because of their understanding, interest, and knowledge of statistical hydrology, mapping, and computer applications, and their ability and willingness to communicate with their colleagues. Members typically served staggered 3-year terms, so a third of the SSAC membership changed annually.

The SSAC developed an initial 5-year national implementation plan for StreamStats in 2005 and has updated it several times over the years. The initial plan recommended solutions for several technical issues, such as server and software strategies and approaches to functionality. The SSAC also produced several policy recommendations for implementation USGS-wide, including (1) assessing local USGS offices to recover part of the development team costs to support implementing StreamStats for the States, (2) encouraging development of applications with geographic boundaries that conformed to river basins rather than to State boundaries, (3) standardizing the names of streamflow statistics and basin characteristics across State lines, (4) mandating reviews of proposals produced by the local USGS offices for implementing StreamStats by the StreamStats development team, and (5) handling of multiple values of the same statistics in StreamStats outputs for streamgages. The SSAC also released an internal report in 2013 that provided a comprehensive approach for how StreamStats implementation could be completed for States where State agencies were unable to provide cooperative funding to support implementation costs. The SSAC has been idle since 2017.

Existing Efforts

A beta version of a time-of-travel client application (https://www.usgs.gov/tools/time-travel-beta-application) for the conterminous United States, based on StreamStats web services, can calculate the travel times between stream locations of real or hypothetical hazardous waste spills and downstream locations of concern based on the equations of Jobson (1996). This application provides maximum probable and most probable travel-time predictions through a pair of tools: the Spill Planning tool and the Spill Response tool.

The Spill Planning tool was designed to help emergency response personnel plan for a potential spill. The tool traces (searches along the stream channel) a specified distance upstream from a user-selected point, often a water-supply intake of interest. After the trace is completed, the user is asked to input a streamflow value, which is then used as input for the Jobson equations, and then the results are presented as a colorized display of accumulated travel times for each stream reach in relation to the selected point.

The Spill Response tool was designed for use in the event of an actual spill. This tool traces a line a specified distance downstream from a user-selected point, which is the location of the spill either on land or on a water feature, to an intake or streamgage of interest. After the line is traced, the user is asked to input a spill mass, recovery ratio (optional), and discharge. Travel times and a peak concentration are then computed for each reach, and the travel times are accumulated in the downstream direction. This tool also offers a line chart depicting travel times for the leading edge, peak, and trailing edge for each reach in the selected area. For large areas, a user may select a stream-reach group to reduce the number of reaches shown in the chart for better visibility.

Output from the time-of-travel application is made available to the user in a final, printable report, with options to include a map of the study area, tables with accumulated maximum probable and most probable travel times, a line chart, a custom title, optional notes, and the citation for Jobson’s (1996) report. Within the output report, the user also has the options to export table content to a CSV file or download spatial data as a GeoJSON file.

For internal modeling uses, the USGS needs StreamStats to have the ability to deliver results based on nationally consistent data, but StreamStats generally was built by using the best available data at the time when it was implemented for each State. A national application has been released in beta format by the USGS at https://streamstats.usgs.gov/national-beta/ that can delineate drainage basin boundaries and compute a small set of basin characteristics (drainage area, percent forest, and January minimum and maximum temperatures) anywhere in the conterminous United States using NHDPlus version 2 datasets and StreamStats toolsets.

The national StreamStats application incorporates a Fire Hydrology tool that provides information needed to assess changes in hydrology after wildfires. Two main effects of wildfires on hydrology are increased runoff and decreased water quality. In the burned area after a wildfire, there is greater potential for flooding, erosion, and stream habitat degradation (Neary and others, 2011). Transportation of sediment, along with increased nutrient and metal concentrations to public water supplies, can lead to higher water-treatment costs and has the potential to affect water availability. The Fire Hydrology tool integrates watershed delineation capabilities with fire perimeter and burn severity layers to calculate postburn peak flows and provides stream tracing to identify affected streams and streamgages. The Fire Hydrology tool can provide actionable intelligence to stakeholders in the wildland fire, local government, and science communities in a streamlined fashion.

A substantially improved replacement has been developed for the Estimate Flows Based on Similar Streamgaging Stations tool from version 2, but it has not yet been released. The version 2 tool provided flow estimates for ungaged sites based on the flow per unit of drainage area for a nearby streamgage, and it worked only when there was a streamgage with a drainage area that is between 0.5 and 1.5 times the drainage area for a selected ungaged site. The new tool removes this limitation and provides indicators of the errors associated with the flow estimates, which generally are lower than the errors provided by the version 2 tool.

Efforts to bridge the gap between hydrology and hydraulics are underway in Massachusetts and South Carolina. Massachusetts has a pilot study to modify StreamStats to deliver peak-flow (unit) hydrographs and then use those hydrographs to solve equations to compute peak flows through culverts at user-selected locations within a major river basin in Massachusetts (U.S. Geological Survey, 2022a). The resulting outputs would include graphical representations and tables of potential dimensions for a culvert at the user-selected location. The South Carolina effort would also deliver unit hydrographs and use the results to compute peak flows using the time-of-concentration method (U.S. Department of Agriculture, Natural Resources Conservation Service, 2021). Other ongoing efforts to enhance StreamStats include development of

Potential Further Improvements

Several enhancements could improve the availability of information from StreamStats and the speed of its delivery. Modifying the StreamStats infrastructure to further optimize the use of cloud technology could deliver better products faster. New capabilities could allow users to get flow-statistic estimates for anywhere within a delineated basin or a specified area rather than just where there are streams, obtain drainage areas for a line across the landscape, such as the edge of a field, or download the underlying data used for a computation.

Further developing the national StreamStats application that provides basin delineations and basin characteristics for user-selected sites using GIS data at a consistent scale nationally will rely on a strong partnership between the StreamStats development team and the USGS National Geospatial Program (NGP), which has a long-term goal of providing a national, seamless, integrated, geospatial framework for hydrologic analysis. With the creation of the 3DHP effort by the NGP, further efforts at developing the NHD and NHDPlus HR have ceased. Expansion of the capabilities of the national application will now rely on use of the 3DEP and 3DHP datasets as they are completed. These datasets will likely be used eventually to replace those used for State applications.

The USGS is developing the National Hydrologic Geospatial Fabric (NHGF) to provide a web-accessible system including a network of connected representations of rivers, lakes, and catchments, derived physical characteristics, analysis tools, and a community of practice for users. The NHGF is being built by using the best available geospatial data, much of it from the NGP. A provisional version of the NHGF system (Blodgett and Johnson, 2022) has been released. It is expected that the NHGF could develop web services through which users could access StreamStats, and the web services developed by the NHGF could be accessed by StreamStats to provide additional functionality.

Improvements to the underlying datasets used for StreamStats State applications would also have many benefits. These improvements would come from increases in the resolution and precision of elevation data derived from lidar and improved methods for hydro-enforcing stream locations through methods such as (1) using selective burning rather than full-stream burning, such as burning only at stream-road crossings; (2) use of precipitation-intensity values to compute volumes of water generated by specified precipitation events, and imposing those volumes on the digital land surface to determine when low areas are likely to contribute surface water (fill-spill volumes); and (3) enforcing surveyed (known) culvert locations. These kinds of improvements to the underlying StreamStats State datasets could likely be achieved through the combined efforts of the StreamStats team, GIS specialists in USGS water science centers, and NGP staff.

The scope and applicability of methods to get flow statistics anywhere could be broadened, particularly with an emphasis on small basins. Currently (2023), more than 50 percent of delineations requested through the StreamStats interface are for basins of less than 2.5 square miles. Because the USGS operates relatively few streamgages with basin areas of this size, regression equations for estimating flow statistics have limited applicability for these small basins. Broadening the use of alternative methods for computing peak flows for small basins, such as the TR–55, the rational, and the time-of-concentration methods, would be helpful to many StreamStats users.

Further efforts to bridge the gap between hydrology and hydraulics could be made, including (1) developing the ability to use multiple resolutions of elevation data, such that higher resolution data could be used near stream channels for hydraulics computations and lower resolution data used elsewhere within a delineated basin; (2) enhancing tools for computing channel cross sections and profiles; (3) adding a tool for computing channel roughness; (4)  linking to databases of hydraulic structures; (5) linking to flood-inundation models; (6) linking to NHGF river corridors and geomorphology data; (7) linking to fill-spill tools for determining volumes needed to spill over roadways; and (8) expanding linkage of basin hydrograph computations with culvert-flow computations.

Additional enhancements could include incorporating flow networks through karst areas and the development of additional transboundary and international applications. Implementation for the Lake of the Woods–Rainy River Basin showcases efforts at harmonization of geospatial data between two nations. Work that has already been completed to harmonize basin boundaries and stream networks for other river basins along the United States-Canadian border, including the Souris River Basin, Milk River Basin, and Kootenai River Basin (all in the western border region), and St. John River Basin (along the Maine border with Quebec and New Brunswick), could be leveraged to implement StreamStats for those basins. Existing methods and infrastructure also could be used to implement StreamStats for other nations.

SMARTStats

The USGS Water Mission Area prepared an internal plan in 2020 to develop an operational process for creating, publishing, and visualizing streamflow statistics that would be computed and released in a nationally consistent manner and on a much more frequent basis compared to past practices, where peak-flow statistics often were computed on average only about once in 10 years and low-flow statistics often were computed even less frequently. The plan would (1) establish a process for computing and routinely updating 60 interpretive and noninterpretive streamflow statistics that are representative of the full range of flows; (2) develop a national streamflow statistics database for the estimated flows and serve these data to the public; (3) use machine learning and hierarchical modeling to regionalize flow statistics; (4) provide estimates of flow statistics for each NHDPlus stream reach in the conterminous United States; and (5) develop a system for publishing relevant visualization products, such as tables and interactive maps, to allow spatial comprehension and analysis of the streamflow statistics. This planned system was named SMARTStats.

The SMARTStats plan would address weaknesses in the StreamStats approach that limit the usefulness of StreamStats-provided information for large, regional analyses. StreamStats relies on a cooperative model for computing streamflow statistics and generating estimates at ungaged sites through use of regression analysis. Consequently, the statistics available through StreamStats are inconsistent in type and currency among the States. Also, over the decades when they have been used by the USGS, the methods of performing regression analyses have improved, so newer machine learning and hierarchical modeling techniques hold promise for delivering superior estimates. Although SMARTStats would address these weaknesses of StreamStats, StreamStats provides substantial additional functionality and custom processes developed for several States that SMARTStats would not address.

A team was established in 2021 to review the SMARTStats plan and identify potential effects of its implementation on internal and external stakeholders, recommend revisions to the plan, and provide an implementation strategy. The review team released 13 findings after evaluating feedback from numerous stakeholder meetings, including that (1) SMARTStats should become part of the ecosystem of services in the planned StreamStats platform architecture, (2) StreamStats should be reconfigured to allow seamless access to both the SMARTStats nationally consistent statistics and those produced by the State offices, and (3) the SSAC should be reconstituted to provide guidance to the team. These recommendations and several others from the review team, as well as the SMARTStats plan itself, will likely have a major effect on how StreamStats operates in the future. The SMARTStats plan did not foresee the development of the 3DHP or the NHGF, and so the plan will likely need to be modified to accommodate the data that will be generated by these programs.