Modeling the Effects of Climate Change on Streamflow and Stream Temperature in the South Fork of the Stillaguamish River

Modeling the effects of climate change on streamflow and stream temperature in the South Fork of the Stillaguamish River Thesis Proposal for the Master of Science Degree, Department of Geology, Western Washington University, Bellingham, Washington Kate Clarke May 2018 Approved by Advisory Committee Members: _____________________________________________________________ Dr. Robert Mitchell, Thesis Committee Chair _____________________________________________________________ Dr. John Yearsley, Thesis Committee Advisor _____________________________________________________________ Dr. Douglas Clark, Thesis Committee Advisor Problem Statement My objective is to model the effects of forecast climate change on streamflow and stream temperatures in the South Fork Basin of the Stillaguamish River in northwest Washington State. I will use gridded historical meteorological data to calibrate the Distributed Hydrology Soil Vegetation Model (DHSVM; Wigmosta et al., 1994) to simulate hydrology and the River Basin Model (RBM; Sun et al., 2014; Yearsley 2009, 2012) to simulate stream temperature. I will then apply gridded forecast climate data that have been downscaled to the region to predict future changes in streamflow and stream temperature through the 21st century throughout the South Fork Basin. I predict that spring and summer streamflow will decrease, and stream temperatures will increase. The Stillaguamish River (Figure 1) is an important regional water resource and serves as critical habitat for several species of salmonids (Washington State Department of Ecology (WSDOE), 2012). The results of my study will help river managers to determine where to focus salmon habitat remediation efforts. Introduction The South Fork Basin encompasses approximately 38% (660 km2) of the Stillaguamish River Basin and serves as an important resource for local agriculture and industry and for habitat for fish (Figure 1). The river is currently subject to a temperature total maximum daily load (TMDL; WSDOE, 2004; SCSWM, 2015), which means that according to the U.S. Clean Water act, it does not meet water quality standards in terms of temperature and must be mitigated. The Stillaguamish Indian Tribe relies on the river for both traditional and economic salmon fishing, so there is concern about the effects of forecasted warming climates on stream temperatures and salmon habitats. In the Pacific Northwest (PNW), global climate models project that the mean air temperature will increase between 3°C to 7°C from late 20th century historic mean temperatures through 2099 (Abatzoglou and Brown, 2012; Mote and Salathé, 2010). Previous studies of similar Puget Sound river basins predict that the forecast increases in average temperature will change precipitation patterns and result in less overall precipitation in the summers and less precipitation falling as snow in the winters (e.g., Cao et al., 2016; Dickerson-Lange and Mitchell, 2014; Murphy, 2016). Earlier snowmelt in the spring and lower streamflow in the summer have been shown to cause higher stream temperatures in Puget Sound rivers (Cao et al., 2016). Cao et al. (2016) modeled the entire Stillaguamish River Basin and found that runoff will increase in the winter and decrease in the summer in response to increased air temperature, increased evapotranspiration, earlier snowmelt, and warming stream temperatures. Field observations and a higher model resolution will allow me to refine and improve on work of Cao et al. (2016). I will use the calibrated DHSVM and RBM to project future streamflows and stream temperatures in response to 20 projected climate change scenarios that have been deemed appropriate for the PNW (Rupp et al., 2013). Modeling results will be used to evaluate threats to fish habitat and will help river managers to assess where to focus habitat remediation efforts. I will also compare my results with the modeling results of Kyra Freeman, who is conducting a similar study in the North Fork of the Stillaguamish River, which is geologically and topographically different than the South Fork Basin. Background Basin characteristics The Stillaguamish River Basin is located mainly in Snohomish County, in northwestern Washington State, and is in Water Resource Inventory Area (WRIA) 5. The Washington Watershed Management Act of 1998 divided watersheds in the state into WRIAs to define a water resource planning framework and management jurisdictions throughout the state (WSDOE, 2000). The bedrock geology of the South Fork Stillaguamish River Basin includes Jurassic metamorphic rocks in the west and Tertiary sedimentary rocks in the east (Benda, 1992). The basin has been substantially shaped by repeated episodes of continental glaciation during the Quaternary Period. The last major advance of the Cordilleran Ice Sheet (ca. 18,000 yr B.P.; Porter and Swanson, 1998) dammed the South Fork, overran much of the basin, then dammed it again as the ice sheet retreated approximately 14,000 years ago. This sequence of events formed thick, extensive glaciolacustrine silts and clays and glaciofluvial sands and gravels along the valley floors and walls throughout much of the basin (Benda, 1992; Booth et al., 2013). These thick sequences of poorly consolidated sediments, combined with a high water table, make the valley susceptible to landslides. The North Fork Stillaguamish River was once the outlet for the nearby upper Skagit River, Suiattle River, and Sauk River during the most recent glaciation of western Washington (Booth et al., 2003). As a result, the main river valley of the North Fork is much wider than the main valley of the South Fork. Due to differences in groundwater contribution to the two forks, the South Fork may respond more drastically to climate change than the North Fork. The National Oceanic and Atmospheric Administration (NOAA) classifies land use within the South Fork of the Stillaguamish River Basin as predominantly forested land with some wetlands and developed land. Timber harvesting has declined in the basin since the 1990s, but the river is still greatly influenced by historic practices, particularly logging of riparian zones, which buffer the stream temperature (Stillaguamish Implementation Review Committee (SIRC), 2005). The climate in the Stillaguamish River Basin is considered maritime, with warm, dry summers and cool, wet winters. This maritime climate classifies the watershed as a rain-snow transitional basin, which is sensitive to climate change (Vano, 2015). Small temperature changes influence whether precipitation will fall as snow or as rain. Surface elevation ranges from about 13 meters near the South Fork mouth to just over 2000 meters at the headwaters near Del Campo Peak. This relatively low elevation range makes the basin particularly sensitive to small changes in winter temperatures, which can change whether precipitation falls as snow or as rain. The position of the basin in the western foothills of the North Cascades results in a steep orographic precipitation gradient. The 30-year normal precipitation means vary between 1.17 meters at low elevations near the South Fork River mouth to about 4.56 m near the high elevation peaks (PRISM Climate Group, 2014). Rainfall runoff contributes to streamflow rapidly, whereas snow is stored and later melts and contributes to streamflow while mitigating stream temperature throughout the spring as air temperatures and day lengths increase (USGS, 2016; WSDOE, 1981). Mean annual discharge of the South Fork at the Washington State Department of Ecology (WSDOE) gauge 05A105 near Granite Falls, WA (Figure 1) is approximately 2440 cubic feet per second (WSDOE, 2018). The highest discharges occur in the fall and winter, while the lowest occur in the dry season between July and September. Climate change in the PNW A general climate warming trend in western Washington has been reported by many studies (e.g., Mote et al., 2014; Mote and Salathé, 2010; Vano et al., 2015). Annual mean temperatures have increased by 0.6°C to 0.8°C from 1901 to 2012. Global climate models project warming of air temperatures by 3°C to 7°C through 2099 in the PNW (Abatzoglou and Brown, 2012). Future trends are expected to increase both the frequency and the intensity of precipitation events in western Washington (Mauger et al., 2016). A 2% to 5% increase per decade of spring precipitation has been observed from 1901 to 2012 (Abatzoglou and Brown, 2012). Climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report predict increases in extreme high precipitation in western Washington and reductions in snowpack in the Cascades (Snover et al, 2013). The University of Washington Climate Impacts Group (UW-CIG) predicts that average spring snowpack in Washington will decrease by 38% to 46% by the 2040s and by 56% to 70% by the 2080s. As a result, seasonal streamflow peaks and patterns will change significantly (Snover et al., 2013). The Stillaguamish River as a fish habitat The Stillaguamish River provides critical habitat for eight salmonid species, three of which have been classified as threatened by the Endangered Species Act since 1999 (SIRC, 2005). The Stillaguamish Tribe depends on the threatened Chinook salmon (Oncorhynchus tshawytscha) as the fish are of high cultural and economic importance. Chinook salmon runs occur once in the summer and once in the fall. The summer runs occur May to September, and the fall runs occur September to December (Kip Killebrew, personal communication, 15 March 2018). Increasing stream temperatures,

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