Fine-grained sediments in the lower reach of the Sauk River are adversely affecting the health and spawning of Chinook salmon. Climate change and forestry practices have been proposed as suspected causes of a reported increase in sediment loading to the river.
To determine the amount and timing of suspended-sediment loading to the river and possible connections to adverse effects on Chinook salmon, the USGS, in cooperation with the Sauk-Suiattle Indian Tribe, will monitor turbidity (an indicator of suspended sediment) and temperature in the river, examine the relation between salmon life cycle and the timing of sediment loading, and identify any land-cover-related causes of increased sediment loading.
Problem - 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). 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 glacier melting from Glacier Peak. Other management-related, sediment sources such as mass-wasting events associated with forestry practices are known to have increased sediment loading in forested mountain watersheds, but the potential contribution from this source in the Sauk River basin is unknown
Objectives - The objectives of the proposed study are to: 1) quantify the contribution of fluvial suspended-sediment load from the Sauk River and its principal tributaries to the Skagit River, 2) document the timing of suspended-sediment transport relative to resident Chinook salmon life-cycles and ambient climatic conditions, and 3) determine the characteristics of fluvial sediments such as size distribution and density, and 4) relate these to sediment production regimes.
Relevance and Benefits - - This study addresses changes in surface-water quality attributable to climate change with direct impacts on aquatic ecosystems; priority issues described in Climate Variability and Change, and Understanding Ecosystems and Predicting Ecosystem Change, as outlined in Facing Tomorrow's challenges: USGS Science in the Decade 2007-2017. The results from this study will help guide future management decisions in the basin by identifying the relative importance of the various sediment/turbidity sources to the rivers, and using that information to prioritize protection and/or restoration activities. This study will contribute important data for climate change scenarios designed to predict impacts of increasing sediment load in glacier-fed rivers that support endangered fish habitat. This study also will contribute to on-going USGS research into sediment delivery to large river deltas in Puget Sound to determine resiliency of delta estuaries versus sea level rise.
Approach - Turbidity and temperature monitoring sites will be established at four river locations in the Sauk River basin, two of which are active real-time streamgaging stations and two of which are discontinued gage sites. The USGS will train tribal staff to assist USGS personnel in the collection of sediment data and sensor maintenance. Turbidity will be developed as a surrogate measurement for suspended-sediment concentration at each site, and turbidity time-series data will be used to calculate suspended-sediment load at various time scales for each site. The timing of suspended-sediment transport and sediment characteristics will be considered relative to fish life cycles, and will be examined with basin land cover to identify likely sediment production regimes.
