Sediment

Sediment

Unknown to many people, the amount of sediment in the Skagit River has important ramifications for water quality (contaminants and turbidity), flooding (reducing channel volume), water supply (sediments and contaminants that attach to sediments must be removed from the drinking supply), agricultural drainage, species survival and hydropower (affects the lifespan of dam storage and turbines).

Sediment quantities and transport have been significantly transformed by the channelization of the Skagit River, human development, and activities such as logging.  Sediment loads are currently 10–35 times the estimated pre-development conditions due to land use change (such as logging and road building), and are expected to increase with climate-induced changes. Channelization of the lower river and the construction of dikes and levees downstream changed the dominant flows path of the Skagit River isolating 90% of the Skagit delta from a high flow deposition area.

Since the North and South Forks of the Skagit River are the only remaining outlets delivering sediments to the delta, the rate of deposition in Skagit Bay has accelerated, creating sand bars, infilling of habitats, and blocking drainage infrastructure. This pattern is more striking in the areas near the North Fork since the dominant flow path was diverted from the South Fork in 1937.   In addition to increasing deposition of sediments in the Skagit delta and Bay the landward areas behind the dikes are slowly sinking creating a situation where humans have to work harder and harder to drain agricultural lands and prevent flooding.

Although overall sediment transport has increased due to development, most sediment that reaches the delta bypasses the shoreline and tidal flats where they would historically remain, and settles instead in deeper waters. Fine sediments are swept away by the currents and settle offshore. The offshore transport of fine sediments as far away as Deception Pass is detrimental to nearshore habitats such as eel grass beds.  Important shrimp and crab areas and rock fish habitat are being buried and fragmented by sediment.  Already, 60–70% of the eelgrass in Skagit Bay has been negatively impacted.  Additionally, fine sediment carries contaminants that are being dispersed further and further afield.

Climate-induced changes in the Skagit Basin are anticipated to increase sediment loads.

More information is available in the recent Northwest Science Special Issue article “Impacts of Climate Change on Regulated Streamflow, Hydrologic Extremes, Hydropower Production, and Sediment Discharge in the Skagit River Basin.”