Sediment Load and Distribution in the Lower Skagit River

abstract

Sediment Load and Distribution in the Lower Skagit River. Submitted to Continental Shelf Research
Curran, C., Grossman, E., Mastin, M. and Huffman, R. (In Review).

Fluvial sediment load and the distribution of water and sediment through principal distributaries are determined for the Skagit River, the largest river entering Puget Sound, in northwest Washington, USA. The Skagit River delivers about 40% of all fluvial sediment to Puget Sound influencing flood hazards in the Skagit lowlands, critically important estuarine habitat in the delta, and some of the most diverse and productive agriculture in western Washington. Using a flow-duration/transport curve approach and 69 years of discharge data (1941–2009), a mean annual suspended-sediment load of 2.6 teragrams (Tg, where 1 Tg = 1 million metric tons) is estimated for the Skagit River near Mount Vernon, Washington, with a range of 1.8 to 5.0 Tg for water years (WYs) 2006-09 (Oct. 1 – Sept. 30). For the relatively ‘wet’ WY 2007, the suspended-sediment load of 4.5 Tg was dominated by a single large flood event that contributed 1.8 Tg, or 40 percent of the annual load. Particle size affects sediment transport, fate and distribution across watersheds and therefore is important for predicting how coastal environments, particularly deltas and beaches, will respond to climate change and sea-level rise. Particle-size analysis of winter storm samples shows suspended sediment is predominantly very fine to medium sand (0.0625–0.5 mm), while bedload (2–3 % of total load) is predominantly medium to coarse sand (0.5–1 mm) for the measured flow conditions. During the summer, much of the fine suspended-sediment (silt and clay particle sizes <0.0625 mm) is generated from glacier melt. Two regression equations relating fine load to suspended-sediment discharge and turbidity are derived and used to estimate the fine load for the period 1941–2009, which ranges from 41 to 46 percent of the total annual suspended-sediment load. The distribution of flow through the delta distributaries is dynamic with twice as much flow through the North Fork than South Fork during low-flow conditions and close to equal flows during higher-flow conditions. Turbidity, monitored at several sites in the lower river in the summer of 2009, was uniform among sites, indicating fines (particle size <0.0625 mm) are well mixed in the lower river system. A strong relationship derived between the concentration of fines and turbidity allows turbidity to be used as an effective surrogate measurement for quantifying fine load, and holds promise for other western Washington rivers that are seasonally dominated by glacier-melt processes. These results provide a comprehensive set of tools to predict sediment delivery and delta responses of interest to scientists and resource managers, including decision-makers examining flood hazards mitigation, estuary restoration, and climate change adaptation.

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