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U.S. GEOLOGICAL SURVEY
Data Series 412

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Introduction

The perennial or intermittent status of a stream has bearing on many regulatory requirements. For example, the application of Total Maximum Daily Load (TMDL) requirements for Idaho depends on whether the relevant stream reach is perennial or intermittent. Because of changing technologies over time, cartographic representation of perennial/intermittent status of streams on U.S. Geological Survey (USGS) topographic maps is not always accurate and (or) consistent from one map sheet to another. This can be problematic because USGS topographic maps often are used to determine the perennial or intermittent status of specific streams. Because of this, the USGS, in cooperation with the Idaho Department of Environmental Quality and the Bureau of Reclamation, is attempting to better estimate and map the perennial and intermittent streams in Idaho.

Idaho Administrative Code IDAPA 58.01.02 (State of Idaho, 2006) defines an intermittent stream as one having an unregulated 7-day, 2-year low flow (7Q₂) of less than 0.1 ft³/s. 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 7Q₂ (Hortness, 2006). Using these regression equations, the 7Q₂ streamflow may be estimated for naturally flowing streams anywhere in Idaho, based on measurements of certain basin and climatic characteristics.

Using this approach to estimate perennial streams, however, in most cases entails using the regression equations far outside the range of values used to develop the regression models. Such extrapolation cannot be avoided because of a lack of stream-gaging station data for such small streams. This can result in unreasonable flow estimates, as well as unreasonable spatial patterns in the modeled stream networks when extrapolating to estimate flows in streams with small drainage areas. As described in Wood and others, 2008, the spatial patterns resulting from continuous estimation of 7Q₂ flows using the equations by Hortness (2006) were used to evaluate regression models for 7Q₂, and alternative models determined to be more appropriate for estimating perennial streams were selected for five of eight low-flow regression regions.

Background

The only sources of information available regarding the perennial or intermittent status of streams often are the USGS 7.5-minute topographic maps for the area in question. Streams shown on these maps have been digitally captured into the National Hydrography Dataset High-Resolution (NHD Hi-Res). According to the (draft) Standards for the National Hydrography Dataset-High Resolution (U.S. Environmental Protection Agency and U.S. Geological Survey, 1999), an intermittent stream “Contains water for only part of the year, but more than just after rainstorms and at snowmelt.” A perennial stream “Contains water throughout the year, except for infrequent periods of severe drought.” Similar standards and procedures were used to develop the National Hydrography Dataset Medium Resolution (NHD Med-Res) from 1:100,000-scale topographic maps. The NHD Med-Res was used as the basis for the NHDPlus, which will be discussed later in this report. In this report, the term “NHD” is used to describe the National Hydrography Dataset in a generic sense when it is not important to distinguish between the NHD Hi-Res and NHD Med-Res.

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 many decades, using varying technologies and standards. Therefore, adjacent topographic maps may have been developed under quite different time frames, using different standards and technologies. Finally, errors sometimes occurred in the process of digitally capturing the topographic map information and incorporating it into the NHD. The NHD originally was processed by 8-digit Hydrologic Units, known as subbasins. The result is that the streams shown in the NHD sometimes reflect unrealistic changes in perennial or intermittent status at quadrangle (topographic map) or hydrologic-unit boundaries. Figure 1 shows the spatial distribution of streams identified as perennial in an area of northern Idaho. Several unrealistic density differences are visible following quadrangle or hydrologic-unit boundaries.

Idaho Definition of Perennial Streams

The State of Idaho does not specifically define perennial streams in its water-quality rules; instead, it applies definitions of ephemeral and intermittent waters. Ephemeral waters are on the drier end of the spectrum of flow from intermittent waters; perennial waters, therefore, are all streams having more flow than intermittent waters. Idaho’s water-quality rules define intermittent waters thus:

Intermittent Waters. A stream, reach, or water body which naturally has a period of zero (0) flow for at least one (1) week during most years. Where flow records are available, a stream with a 7Q₂ hydrologically-based unregulated flow of less than one-tenth (0.1) cubic feet per second (cfs) is considered intermittent. Streams with natural perennial pools containing significant aquatic life uses are not intermittent. (IDAPA 58.01.02.010.45)

This project focused on the 7Q₂ less than 0.1 ft³/s criterion as one that can be estimated from existing flow records at established gaging stations. The 1-week duration of no flow criterion and the possible persistence of perennial pools in non-flowing streams were not used in this project. The definition specifies that the basis for classifying streamflow is the natural (unregulated) flow; therefore, this project was concerned with classifying stream segments as perennial or non-perennial based on estimates of unregulated 7Q₂. This classification is of interest to the State of Idaho because permanence of flow has bearing on monitoring and methods of assessing water quality, application of water quality standards, and determination of appropriate use designations, and affects the extent of authority under the Clean Water Act to regulate water quality.

Purpose and Scope

This report describes the datasets and procedures used to create a geospatial dataset of perennial streams in Idaho. A by-product of the effort to model perennial streams is an integrated suite of geospatial datasets derived from 10-m resolution Digital Elevation Models (DEMs) and 1:24,000-scale NHD. These improved datasets also are described in this report, and will be incorporated into the Idaho StreamStats application in the future. (More information about StreamStats is available at http://streamstats.usgs.gov/.) This report provides links to Federal Geographic Data Committee (FGDC)-compliant metadata (Federal Geographic Data Committee, 1998) that describe these new geospatial datasets and to distribution information that describe how to obtain the datasets.

Some shortcomings of previous datasets of perennial streams in Idaho and qualitative improvements in the spatial patterns of modeled perennial streams are described. Quantitative analysis of these data improvements by comparison to field data is described by Wood and others (2008).

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