Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 1156
Title: Hydrogeomorphic classification of Washington State rivers to support emerging environmental flow management strategies
Author: Catherine A. Reidy Liermann, Julian D. Olden, T. J. Beechie, Mark J. Kennard, Peter Skidmore, Chris Konrad, H. Imaki
Publication Year: 2011
Journal: River Research and Applications
Volume: 28
Issue: 9
Pages: 1340-1358

As demand for fresh water increases in tandem with human population growth and a changing climate, the need to understand the ecological tradeoffs of flow regulation gains greater importance. Environmental classification is a first step towards quantifying these tradeoffs by creating the framework necessary for analysing the effects of flow variability on riverine biota. Our study presents a spatially explicit hydrogeomorphic classification of streams and rivers in Washington State, USA and investigates how projected climate change is likely to affect flow regimes in the future. We calculated 99 hydrologic metrics from 15 years of continuous daily discharge data for 64 gauges with negligible upstream impact, which were entered into a Bayesian mixture model to classify flow regimes into seven major classes described by their dominant flow source as follows: groundwater (GW), rainfall (RF), rainwithsnow (RS), snowandrain (SandR), snowwithrain (SR), snowmelt (SM) and ultrasnowmelt (US). The largest class sizes were represented by the transitional RS and SandR classes (14 and 12 gauges, respectively), which are ubiquitous in temperate, mountainous landscapes found in Washington. We used a recursive partitioning algorithm and random forests to predict flow class based on a suite of environmental and climate variables. Overall classification success was 75%, and the model was used to predict normative flow classes at the reach scale for the entire state. Application of future climate change scenarios to the model inputs indicated shifts of varying magnitude from snowdominated to raindominated flow classes. Lastly, a geomorphic classification was developed using a digital elevation model (DEM) and climatic data to assign stream segments as either dominantly able or unable to migrate, which was crosstabulated with the flow types to produce a 14tier hydrogeomorphic classification. The hydrogeomorphic classification provides a framework upon which empirical flow alteration–ecological response relationships can subsequently be developed using ecological information collected throughout the region.

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