Isoscapes at the River Basin Scale
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Climate fluctuation affects the timing, magnitude and spatial distribution of precipitation and streamflow across the continent. Stable isotopes are a conservative tracer of hydrologic flows, and integrate spatial information about the water’s origin. Stable isotopes of precipitation vary spatially, allowing for the creation of isoscapes: a map of the spatial signatures of precipitation. This tracer can be used to understand which regions of a watershed are contributing water to larger integrated rivers, and how those water sources are changing over time. We used isoscapes to help understand how and when water sources are changing within major water bodies in the Pacific Northwest to identify hydrologically important parts of the landscape. Across the Pacific Northwest, stable isotopes of water in small headwater streams vary strongly with elevation and provide a fingerprint for determining the mean elevation from which water in rivers is derived. Much of the water that people in the Pacific Northwest rely on comes from snowpack in the Cascade Range or other nearby mountain ranges, and their snowpacks are expected to decrease in coming years. In fact, the past 8 years have shown dramatic variation in snowpack, from a high of 174% of normal in 2010-11 to a low of 11% for 2014-15, one of the lowest on record. We developed isoscapes for the following basins: Willamette and Marys Rivers in Oregon, Snake River in Idaho, Snoqualmie, Green, Skagit and Wenatchee Rivers in Washington, and Cowee River in Alaska. We explore temporal trends in water originating from these high elevation snowpack areas to understand the impact of diminishing snowpack for the region and find not all basins are created equal. The underlying geology plays a strong role in determining watershed vulnerability. Reconciling the demands between human use and biological instream requirements during summer will be challenging under climatic conditions in which winter snowpack is reduced compared to historical amounts