A SUSPENDED SEDIMENT BUDGET FOR A GLACIER-FED RIVER: SAUK RIVER, WASHINGTON, USA

The Sauk River, which drains a 1,900 km² area of the North Cascade Range including Glacier Peak, is one of the few remaining large, glacier-fed rivers in western Washington that is unconstrained by dams and drains a relatively pristine landscape. In Washington State, glaciated stratovolcanoes like Glacier Peak are a dominant source of regional sediment loads and have significant influences on the morphology, flow-conveyance, and ecosystems of downstream rivers and estuaries.

Suspended-sediment and turbidity monitoring, conducted over a five-year period from October 2011 to September 2016, provided the first-ever estimates of suspended-sediment load for the Sauk River and documented both the episodic nature and seasonality of sediment-production regimes inherent in glacier-fed rivers. The study design allowed comparisons between the suspended-sediment yields from a non-glaciated subbasin and glaciated subbasins of two principal tributaries, the White Chuck and Suiattle Rivers, which originate at Glacier Peak. During the study period, annual suspended-sediment load (SSL) for the Sauk River ranged from 400,000 to 2,500,000 metric tons (t) with a mean of 950,000 t, and more than half of annual SSLs occurred during the fall season (September to December). The largest single-day SSL (400,000 t) accounted for 16 percent of the annual SSL in the 2016 water year, and corresponded with a 6.4-yr discharge (1,495 m³/s), the result of an intense November storm. About 80 percent of SSL in the lower Sauk River originated from the Suiattle River and suspended- sediment yield for the Suiattle subbasin was 680 t/km²/yr, more than twice the yields for the upper Sauk (non-glaciated) and White Chuck subbasins which were 240 and 300 t/km²/yr, respectively. About 60 percent of SSL in the Suiattle River is attributed to geomorphic activity on the east flank of Glacier Peak. Some of the highest suspended-sediment concentrations recorded were the result of an outburst flood and associated debris flow, but more typically were observed during the first fall storms when glacially-derived sediment that had accumulated in and along the river was flushed downstream. As warming trends drive glacier retreat, baseline knowledge of sediment loads in glacier-fed rivers is needed to understand influences on downstream morphology and ecosystems.