2013: Suspended-Sediment Concentration in the Sauk River, Washington, Water Years 2012-13
Christopher A. Curran, Scott Morris, and James R. Foreman
The Sauk River is one of the few remaining large, glacier-fed rivers in western Washington that is unconstrained by dams and drains a relatively undisturbed landscape along the western slope of the Cascade Range. The river and its tributaries are important spawning ground and habitat for endangered Chinook salmon (Beamer and others, 2005) and also the primary conveyors of meltwater and sediment from Glacier Peak, an active volcano (fig. 1). As such, the Sauk River is a significant tributary source of both fish and fluvial sediment to the Skagit River, the largest river in western Washington that enters Puget Sound. Because of its location and function, the Sauk River basin presents a unique opportunity in the Puget Sound region for studying the sediment load derived from receding glaciers and the potential impacts to fish spawning and rearing habitat, and downstream river-restoration and floodcontrol projects. The lower reach of the Sauk River has some of the highest rates of incubation mortality for Chinook salmon in the Skagit River basin, a fact attributed to unusually high deposition of fine-grained sediments (Beamer, 2000b). The Suiattle River, a principal tributary to the Sauk River, is similarly impaired by sediment and Chinook have been observed to favor spawning sites in the relatively clearer mouths of tributary creeks (Beamer and others, 2005). In recent years the volcano’s glacier-sediment pulses have appeared to coincide with spawning runs in late spring and summer, and the effects of this on fish health during the fry to smolt stages is unknown. The 2005 Skagit River Recovery Plan asserts that levels of sedimentation in the lower Sauk River have increased since 1991 (though data to support this is limited) and attributes this to accelerated glacial melting from Glacier Peak. Other, managementrelated, sediment sources such as mass-wasting events associated with forestry practices have increased 2 sediment loading in forested mountain watersheds, but the potential contribution from this source in the Sauk River basin is unknown. Increased sediment delivery from the Sauk River has downstream implications for human safety (flood control), economic interests (agriculture) and fisheries management associated with the Skagit River, which is Puget Sound’s largest river and fish producer. In cooperation with the Sauk-Suiattle Indian Tribe, the U.S. Geological Survey (USGS) was asked to monitor turbidity and calculate suspended-sediment concentrations and loads in the Sauk River beginning in October, 2011. During the project, USGS operated turbidity sensors at established streamflow gaging stations at three locations in the Sauk River (fig. 1) and collected suspendedsediment samples over a range of turbidity and flow conditions at each site. A relation between turbidity and suspended-sediment concentration was developed at each site, consistent with established USGS protocols (Rasmussen and others, 2009), to enable reporting of 15-min suspended-sediment concentration and calculation of daily suspended-sediment loads.