The Puget Sound basin includes at least thirteen major river systems and numerous tributaries, which can be classified as rainfall-dominated, snowmelt-dominated, or transitional (Ross 2006, Cayan 1996, Bach 2002). Rainfall-dominated rivers exhibit peak flows during winter; snowmelt-dominated rivers have peak flows in late spring and late fall with low winter flows. Transitional rivers exhibit less pronounced high or low flows in the late fall and late spring, and winter. Hydrologic flow patterns are important both ecologically and in terms of consumptive resources. Alteration of historic flow patterns may cause ecological harm and supply disruptions (Wiley and Palmer 2008, Poff et al. 1997). Hydrologic flow regimes in Puget Sound rivers have been altered through the construction of dams for flood control or power generation, or by changes in land cover and climate. Flows in the Skagit, Nisqually, Green, Skokomish, and Cedar rivers are regulated by dams (Puget Sound Partnership 2009c).
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Overview
A 2018 report from the University of Washington Puget Sound Institute analyzes trends in summer stream flows and finds they are declining, but not necessarily because of abstractions by humans.
![All scenarios project warming for the 21st century. The graph shows average yearly temperatures for the Pacific Northwest relative to the average for 1950-1999 (gray horizontal line). The black line shows the average simulated temperature for 1950–2011, while the grey lines show individual model results for the same time period. Thin colored lines show individual model projections for two emissions scenarios (low: RCP 4.5, and high: RCP 8.5)[ ], and thick colored lines show the average among models projecti](/sites/default/files/styles/large/public/topical_article/images/TempChangeGraph.png?itok=kXP4zi8H "All scenarios project warming for the 21st century. The graph shows average yearly temperatures for the Pacific Northwest relative to the average for 1950-1999 (gray horizontal line). The black line shows the average simulated temperature for 1950–2011, while the grey lines show individual model results for the same time period. Thin colored lines show individual model projections for two emissions scenarios (low: RCP 4.5, and high: RCP 8.5)[ ], and thick colored lines show the average among models projections for each scenario. Bars to the right of the plot show the mean, minimum, and maximum change projected for each of the four emissions scenarios for 2081-2100, ranging from a very low (RCP 2.6) to a high (RCP 8.5) scenario. Note that the bars are lower than the endpoints from the graph, because they represent the average for the final two decades of the century, rather than the final value at 2100. Figure source: Climate Impacts Group, based on climate projections used in the IPCC 2013 report.")
