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U.S. GEOLOGICAL SURVEY
Data Series 412

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Summary and Conclusions

Regionalized linear regression models for estimating the 7-day, 2-year low flow (7Q₂) statistic with a 0.1 ft³/s cutoff criterion may be used to estimate perennial streams in Idaho. Regression equations were applied to create continuous grids of 7Q₂ estimates for eight regression regions of Idaho. By applying the 0.1 ft³/s criterion, the perennial streams have been estimated in each low-flow region. To measure estimate uncertainty, a “transitional” zone was identified that corresponded to flow estimates of 0.1 ft³/s plus and minus one standard error. Although the regression models provide overall estimates based on the general trends within each regression region, additional estimate uncertainty exists. Local factors such as a large spring or a losing reach are not included in these regression models, and these factors may greatly affect flows at any given point. Site-specific flow data with a sufficient period of record generally would better represent flow conditions at a given site than flow estimates based on regionalized regression models. The geospatial dataset of perennial streams of Idaho is considered a first-cut estimate, and should not be construed to override site-specific flow data.The modeled perennial streams are provided in this report in two different forms: a shapefile of estimated synthetic perennial streams, and an event table that may be used with the flowline route system in the NHDPlus Version 01_02. Additional datasets designed to work with the ArcHydro Tools also are included. These datasets may be used to delineate watersheds, compute basin characteristics, and compute streamflow estimates throughout the state of Idaho. These datasets will be used by the Idaho StreamStats web application in the future, and represent a significant improvement over the data currently being used for Idaho StreamStats.

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