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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2009–5015

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Introduction

The perennial or intermittent status of a stream has bearing on many regulatory requirements. For example, the classification of flow as perennial or intermittent in a stream reach is considered in the application of Total Maximum Daily Load requirements for Idaho. Additionally, permanence of flow has bearing on monitoring and methods of assessing water quality, application of water quality standards, and determination of appropriate use designations. U.S. Geological Survey (USGS) topographic maps commonly are used to determine the perennial or intermittent status of stream. However, there is a general recognition that the cartographic representations of perennial and intermittent streams on USGS topographic maps are not as accurate or consistent as desirable from one map sheet to another. As a result, the USGS, in cooperation with the Idaho Department of Environmental Quality (IDEQ) and Bureau of Reclamation, is attempting to better define the perennial and intermittent status of streams in Idaho.

Idaho Administrative Code defines an intermittent stream as one having a hydrologically-based, unregulated 7-day, 2-year low flow (7Q₂) of less than 0.1 ft³/s (State of Idaho, 2006). The 7Q₂ is the annual minimum mean streamflow over 7 consecutive days that has a 50 percent probability of not being exceeded in any one year. The USGS has developed regional regression equations for Idaho streams for several low-flow statistics, including the 7Q₂ statistic (Hortness, 2006), using long-term streamflow-gaging station data. Using these regression equations, the 7Q₂ streamflow may be estimated for naturally flowing streams in most areas of Idaho, based on estimates of certain basin and climatic characteristics. The study area and hydrologic regions for which equations were developed are shown in figure 1. The boundary of region 6 was extended in the Hortness (2006) study to include streamflow-gaging station data in Montana; however, for this study, the equations were applied only within the Idaho State boundary in region 6. The undefined region shown in figure 1 represents the eastern Snake River Plain, which includes several dams, major irrigation diversions, springs, and drainages and channel bottoms with high infiltration rates. Due to these conditions, flows in this area could not be characterized using a regional regression approach and are not included in this analysis.

The best sources of information available regarding the perennial or intermittent status of streams often are the USGS 1:24,000-scale topographic maps for the area in question: perennial streams are shown on these maps as solid blue lines, and intermittent streams are shown as dot-dashed lines. Streams from these maps have been digitally captured into the National Hydrography Dataset High-Resolution (NHD Hi-Res). According to the draft standards for the NHD Hi-Res, an intermittent stream “contains water for only part of the year, but more than just after rainstorms and at snowmelt”, and a perennial stream “contains water throughout the year, except for infrequent periods of severe drought” (U.S. Environmental Protection Agency and U.S. Geological Survey, 1999). Similar definitions were applied in developing the NHD Medium-Resolution, which generally was based on 1:100,000-scale USGS topographic maps.

Although the information represented on USGS topographic maps generally was field verified at the time of map compilation, it was not always possible to verify the perennial or intermittent status of every stream. Additionally, the various topographic maps were compiled over a period of many decades, using varying technologies, cartographers, and standards. Therefore, adjacent topographic maps might have been developed at different times, using different techniques, resulting in mapped streams that change perennial or intermittent status or density where they cross map quadrangle boundaries. Differing techniques and standards also were used and mistakes sometimes were made in the process of digitally capturing the topographic map information and incorporating it into the NHD, which was compiled under 8-digit Hydrologic Unit Codes (HUCs). As a result, streams in the NHD often show changes in patterns at HUC boundaries. The spatial distribution of streams identified as perennial according to the NHD in an area of northern Idaho is shown in figure 2. Several non-realistic density differences are visible following 1:24,000-scale quadrangle and HUC boundaries.

Purpose and Scope

The purpose of this report is to describe the procedures used to create a geospatial model of perennial streams in Idaho and to assess the validity of the model. A by-product of the effort to model perennial streams is an improved suite of geospatial datasets derived from 10-m resolution Digital Elevation Models (DEMs). A full description of the geospatial datasets generated and links to Federal Geographic Data Committee-compliant metadata are provided in Rea and Skinner (2009).

This report describes some shortcomings of previous datasets of perennial streams in Idaho and shows qualitative improvements in the spatial patterns of modeled perennial streams. Quantitative analysis of these data improvements by comparison to field data also is presented.

Description of Study Area

The study area comprises the entire state of Idaho and parts of Washington, Oregon, Nevada, Utah, Wyoming, and Montana where hydrologic basins cross state boundaries (fig. 1). The area is divided into eight hydrologic regions and one undefined region, based on geographic features and a cluster analysis of streamflow data, as described in Hortness and Berenbrock (2001) and Hortness (2006). Terrain is highly varied across the study area, with rugged, mountainous areas in the central and northern areas and gently sloping plains, hills, and canyons in the south and western panhandle of Idaho. Elevations range from 738 ft in Lewiston (western panhandle) to 12,655 ft at Borah Peak (central mountains). Geologic features generally consist of igneous, metamorphic, and sedimentary rocks ranging in age from Precambrian to Holocene (Bond, 1978). The southern and western parts of the study area are dominated by basalts, and the granitic Idaho batholith is the major geologic feature in the central and far northern areas (Ross and Savage, 1967). Glacial erosion and deposition have molded the landscape in mountainous areas and in the northern tip of Idaho’s panhandle (Link and Welhan, 2002).

Annual precipitation varies widely with topography, ranging from less than 10 in. in south-central Idaho to 60 to 70 in. in the central mountains (Molnau, 1995). Generally, the source of moisture for precipitation is the Pacific Ocean, although many summer thunderstorms in eastern Idaho are generated by moisture-laden air moving north from the Gulf of Mexico and the Caribbean (National Oceanic and Atmospheric Administration, 1985). About 60 percent of annual precipitation falls during winter and early spring (October through March) as a result of orographic effects (Molnau, 1995); however, summer thunderstorms can contribute significantly to total annual precipitation in the southeastern part of the study area. Highest streamflows generally are observed in April, May, June, and July as a result of snowmelt and rain-on-snow storm events. Baseflow in most unregulated streams dominates flow during August through March, and annual minimum streamflows typically occur in October through January (Hortness, 2006).

Idaho Definition of Perennial Streams

The State of Idaho defines intermittent and ephemeral streams, but not perennial streams, in its water quality rules. Intermittent waters are defined as “a stream, reach, or water body which naturally has a period of zero flow for at least 1 week during most years” (State of Idaho, 2006). Ephemeral waters are on the drier end of the spectrum of flow from intermittent waters, and perennial waters are, by default, all streams with flow greater than meets the definition of intermittent waters. Where flow records are available, a stream with a hydrologically-based, unregulated 7Q₂ flow of less than 0.1 ft³/s is considered intermittent. Streams with natural perennial pools containing significant aquatic life uses are not intermittent (State of Idaho, 2006). Because the presence of natural perennial pools is a site-specific determination, this project focuses only on the unregulated 7Q₂ flow less than the 0.1 ft³/s criterion for perennial stream classification.

Related Studies

A number of investigations have been conducted to classify or map intermittent and perennial streams, or both, often based on state-specific definitions and in response to observed deficiencies in existing maps. Many approaches rely on site-specific field determinations to classify streams, which may be time consuming and costly but accurate if proper and consistent techniques are applied. Recent approaches have relied on statistical techniques and GIS or remotely-sensed data to predict the locations of intermittent and perennial streams.

A comprehensive study involving field techniques was completed by the State of North Carolina. The North Carolina technique requires trained field technicians to score and classify a stream site based on hydrologic, geomorphologic, and biologic metrics (North Carolina Division of Water Quality, 2005). The technique is designed to classify a site based on long-term indicators of perennial or intermittent flow, not simply on the presence of flow at the time of visit. The U.S. Environmental Protection Agency (USEPA) has developed a similar field operations manual for assessing hydrologic permanence in headwater streams (Fritz and others, 2006). The U.S. Army Corps of Engineers and USEPA currently are combining elements of these methods into a guidance document for classifying streams in Oregon (Brian Topping, U.S. Environmental Protection Agency, oral and written commun., 2007). A similar technique has been adopted by the State of Virginia, Fairfax County (Fairfax County, Virginia, Stormwater Planning Division, 2004), although their technique has been adapted to account for seasonal variations and to include anecdotal or historical information in the final scoring of the site.

Jaeger and others (2007) conducted a study in southwest Washington State to field map channel heads and perennial flow initiation points in streams in three test watersheds. They designated areas with continuous and discontinuous flows along stream reaches over a range of hydrologic conditions from February to September 2003. The perennial flow initiation point was identified as the farthest upslope location with flow during baseflow conditions (that is, summer). They determined that the relation between drainage area and perennial flow initiation varied by lithology. For example, the initiation point migrated seasonally to a much greater extent when underlying lithology was sandstone rather than basalt; as a result, the relation between drainage area and perennial flow for all lithologic types was not well-defined. From a practical standpoint, Jaeger and others (2007) recommended establishing minimum drainage areas for each lithology (for example, 0.004 mi² for sandstone and 0.0008 mi² for basalt) as a conservative management strategy for identifying and mapping perennial flow initiation points on a broader scale.

Clarke and others (2008) combined regional GIS data and field observations to delineate a high-resolution, synthetic stream network and hydrogeomorphic attributes in coastal Oregon. The resulting map and dataset have served various uses—from management of riparian forests to habitat evaluation for salmonids. The probability of perennial flow in a stream was one of many stream attributes modeled. The probability was based on drainage area, developed using a dataset from the Siuslaw National Forest (SNF) where field technicians determined the upper limit of perennial flow in selected streams during baseflow conditions over a 2-year period. Clarke and others (2008) assumed that, in drainage areas between 0.004 and 0.14 mi², probability of perennial flow in a stream reach will follow a cumulative distribution function developed using the SNF field dataset. For example, a 70 percent probability of perennial flow is assigned to stream reaches with a drainage area of 0.015 mi² because 70 percent of the stream reaches in the field dataset at that drainage area had perennial flow. These probabilities were used to generate a map of synthetic perennial streams.

Bent and Steeves (2006) used a statistical and GIS-based approach to predict the locations of perennial streams in Massachusetts. A logistic regression equation was developed by relating field observations of the presence or absence of flow at 351 unregulated stream sites with selected basin characteristics such as drainage area, areal percentage of sand and gravel deposits, areal percentage of forest cover, and whether the site was in the eastern or western region of the state. Sites with drainage areas greater than 2.00 mi² were assumed to flow perennially and were not used in the logistic regression equation. Bent and Steeves (2006) then used a GIS automated mapping procedure to calculate the explanatory variables in the logistic regression along a stream reach and determine where the transition from intermittent to perennial flow occurs, based on a probability threshold. They determined that the logistic regression equation correctly classified intermittent/perennial flow at 80.3 percent of streams visited for verification with drainage areas from 0.00 to 10.99 mi², but that USGS topographic maps portrayed the correct stream classifications at only 69.2 percent of the same verification sites. Olson and Brouillette (2006) developed a similar logistic regression equation to predict the locations of intermittent streams in Vermont. The equation was developed by relating field observations from 682 unregulated stream sites to basin characteristics such as drainage area, elevation, ratio of basin relief to basin perimeter, and areal percentage of well- and moderately well-drained soils. Their procedure correctly classified intermittent/perennial flow at 85 percent of streams visited for verification in Vermont.

Many of these efforts to predict the locations of intermittent and perennial streams have demonstrated the usefulness of larger-scale, statistical and GIS-based techniques combined with field verification data over a range of hydrologic conditions. Studies based solely on field visits often are hampered by time limitations. As a result, a range of hydrologic conditions is not always captured in the study, and therefore, final stream classifications may be biased.

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