Estimating Regional Patterns and Drivers of Flow Alteration in U.S. Pacific Northwest Rivers and Streams Based on Channel Morphology from EPA’S National Aquatic Resource Surveys
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As in many regions of the U.S., alterations in the flow of Pacific Northwest rivers and streams have great potential to adversely affect physical habitat, water quality, and biological assemblages in these waters. The magnitude, frequency and timing of stream flows can be altered by flood and low flow management, changes in drainage basin impervious area, extraction of surface and groundwater, and by natural or anthropogenic changes in precipitation or evaporation. We calculated several indicators of hydro-alteration using field measurements of channel morphology routinely collected at sites sampled by the USEPA’s National Aquatic Resource Surveys (NARS). By truthing these estimates against USGS gauges and NARS snapshot discharge measurements, we were able to expand on the range of stream and river sizes where discharge measurements were available. This enabled us to estimate hydro-alteration in all the NARS sites, where comprehensive measures of biota, physical habitat, and chemistry are also made. Our predictions of low-flow discharge (Log-transformed) had regression RMSE=0.54 compared with actual low flow discharge measurements (m3/s and m3/s-km2) ranging over six orders of magnitude at >300 closely matched USGS gauge locations in the U.S. At the same locations, we predicted bankfull floods (Log-transformed) that recur at intervals ranging between 1.3 and 2.5 yr, with regression RMSE=0.50. The primary drivers of discharge in Pacific Northwest river and stream sample sites were drainage area, precipitation, and temperature; but we demonstrated altered summer low and bankfull flows associated with the general level of anthropogenic activity. Diminished summer low and bankfull flows were primarily associated with agricultural land use, particularly when located in or near riparian areas, whereas urban land uses were associated with augmented summer and bankfull flows. Our approach offers promise for supporting the evaluation of the effects of hydrologic alteration on physical habitat and biota in rivers and streams.