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

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.

Impact/Purpose

Flow alterations in rivers and streams continue to be a major cause of habitat degradation and impairment of water quality and biotic integrity in the Pacific Northwest and other regions of the U.S. Unfortunately, EPA’s National Aquatic Resource Surveys (NARS) do not yet have direct measures that would allow us to monitor the status and trends in this important stressor. The authors propose a practical approach for regional and national scale assessment and monitoring of hydrologic alteration. Critical requirements of their approach are evaluations of the precision and accuracy of flow estimates calculated from channel habitat data routinely collected by NARS at thousands of stream and river sampling sites throughout the U.S. The authors compare summer low flow and bankfull flow estimated from channel size, slope, wood, and residual depth measured by NARS with long-term flow measurements from closely-matched USGS gauge locations in the U.S. They then contrast likely geoclimatic and anthropogenic factors controlling stream flows in the Pacific Northwest with those in other regions throughout the U.S. Current results suggest that indicators for assessment of hydro-alteration within the rigorous NARS statistical design may be ready for application in the next rivers and streams assessment. Because these indicators are based on physical habitat data currently collected by NARS, or by State and local agencies using comparable methods, we expect that analytical ability to examine status and trends from 2008-09 through 2018-19 will be ready for EPA’s next National Rivers and Stream Assessment and other state and regional monitoring programs collecting comparable physical habitat data.

Citation

Kaufmann, Phil. 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. Pacific Northwest Chapter of the Society for Freshwater Science, NA, ID, November 18 - 19, 2020.