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

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Summary

The U.S. Geological Survey (USGS), in cooperation with the Idaho Department of Environmental Quality (IDEQ) and Bureau of Reclamation, developed a map of streams with perennial flow in eight hydrologic regions in Idaho using generalized least-squares (GLS) regression models for estimating the 7-day, 2-year low flow (7Q₂) statistic and applying a 0.1 ft³/s cutoff criterion. The regression equations and perennial streams model are applicable only to unregulated, naturally-flowing streams. A transition zone was modeled as plus or minus 1 standard error around the 0.1 ft³/s criterion to account for changes in the perennial flow initiation point due to climatic, hydrologic, and statistical variability. In many cases, it was necessary to apply the regression equations well below the range of values used to develop the regression models. This extreme extrapolation produced unrealistic flow estimates and spatial patterns in the modeled stream network in the headwaters of some regions. As a result, alternative regression models for 7Q₂ were generated for five of eight regions to produce realistic results in the range of extreme extrapolation. Standard errors of prediction were calculated for the regression equations to provide a measure of overall error and predictive power of the model within the range of input variables. Standard errors of prediction for the regression equations were lowest for region 6 (+75.0 to -42.9 percent) and highest for region 7 (+277 to -73.5 percent). Although the equations published by Hortness (2006) remain the preferred equations to use for generating streamflow statistics within the range of the original input variables, the selected alternative equations are preferred for extrapolating estimates of the 7Q₂ in small drainage areas for the purpose of determining perennial flow initiation points.

The resulting 7Q₂ equations were used in conjunction with a continuous parameter grid of basin characteristics to generate a geographic information system-based map of streams in Idaho with perennial flow according to the state definition of intermittent flow as less than 0.1 ft³/s. The final map of perennial streams consists of synthetic lines or vector representations of streams from the gridded model and is provided in Environmental Systems Research Institute shapefile format.

The perennial streams model was evaluated by comparing classifications at various stream points on the map with four independent datasets from government agencies: IDEQ Beneficial Use Reconnaissance Program (BURP), Environmental Monitoring and Assessment Program (EMAP) Office, EMAP Field, and USGS Low Flow Network datasets. Overall, 81 percent of the comparison data points agreed with the USGS model. As a whole, the USGS model appeared to overestimate the amount of perennial streams in comparison with the independent datasets, meaning that many streams were predicted perennial when comparison datasets classified them as intermittent. Some disagreements between the model and comparison datasets were due to poor site location coordinates, timing of the comparison site visits during unusually wet or dry years, discrepancy between 1-day site visits and a 7-day low flow criterion, localized groundwater contributions to flow in some areas, and uncertainty due to model extrapolation. The IDEQ BURP dataset represented a range of climate conditions, including about equal numbers of wet and dry years, and included the highest number of comparison sites. Therefore, the BURP dataset is considered the most representative dataset for comparison, and it agrees well with the USGS model (85 percent agreement overall). Overall, regions with the highest number of disagreements have a high percentage of mountainous and forested area. The regions also include many mountain front recharge zones where streamflow losses to groundwater typically occur, possibly resulting in a change from perennial to intermittent flow. In these areas, the USGS model over-predicted the number of perennial streams in comparison with the National Hydrography Dataset and independent datasets. The USGS model does not account for these site-specific hydrologic conditions and may not adequately represent true conditions in stream reaches where groundwater gains and losses are highly variable. Regions with the highest agreements have a high percentage of low gradient, low elevation area and fewer mountain front recharge zones, resulting in more consistent low flow patterns that were well represented by the GLS regression equations. Site-specific flow data, assuming a sufficient period of record, generally would be considered to better represent flow conditions at a given site than flow estimates based on regionalized regression models; however, flow data on small headwater streams rarely are available and not always practical to obtain. The developed map is considered to be a first-cut, broad scale estimate of the locations of streams with perennial flow.

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