Grossman Sediment Budget Skagit Delta

Sediment Budget for the Skagit Delta: Forecasting Coastal Resilience to Climate Change

EPA Region 10 Scientific Investigations Grant: $100,000 total funding from 2010 to 2012
Additional Funds Needed for Completion: $300,000 (three years)

Top-map of the extensive (16.8 Mm³) sediment deposit that formed between 1940 and 2008 at the Skagit Delta front, which fragmented eelgrass, habitats, and marine resources as it bypassed the delta. Bottom-photograph of the sediment-rich river plume entering Skagit Bay.

Background and motivation: The Skagit River Delta provides diverse habitats for salmon, forage fish, birds, and people. The delta’s residents face important decisions to mitigate flood hazards, recover ecosystem functions, and manage population growth. Recent research on the sediment budget of the Skagit River indicates that much of the sediment delivered to the delta is lost offshore as it is focused through channelized dike complexes between Mt. Vernon and the shoreline. Since the 1850s, the amount of sediment bypassing the delta has increased, burying and fragmenting valued nearshore habitats such as eelgrass meadows, and reducing the diversity and biomass of marine resources important as food-prey for endangered salmon, forage fishes, birds, and humans. This exported sediment is a lost resource for the delta to mitigate sea-level rise. Expected climate changes that affect precipitation and air temperatures will accelerate the retreat of snowpack and glaciers and are likely to increase alpine mass wasting and overall sediment delivery through the Skagit River. This will affect aggradation of the river channel bed that strongly influences flood hazard, patterns of sediment deposition and erosion, habitats, and risk to human infrastructure. Local and regional governments and organizations, including the Sediment Management Issues Group of the Washington Association for Floodplain Management), are currently working to identify sediment management solutions that will balance the needs of flood hazard, ecosystem restoration, and expected population growth. This project provides the quantitative data to determine sediment budgets and the physical processes affecting sediment transport and fate. This information is essential to predict threats to human lives and changes to the infrastructure, habitats and ecosystems, upon which humans depend for livelihood.

Objective: The objectives of this research are to (1) quantify a sediment budget for the delta and (2) determine how sediments that are delivered to or exported from the delta will be modified by climate change and influence: (a) coastal and nearshore flood hazard, (b) the ability of the delta to accrete or retain sediment and keep pace with sea-level rise, and © habitat structure across the delta that supports valued ecosystem components (seagrasses, forage fish, shellfish, birds), recreation, and cultural uses.

Project Team: The project team consists of scientists of the U.S. Geological Survey, Skagit River System Cooperative (SRSC), and The Nature Conservancy (TNC).

Expected outcomes (2011–2012): A sediment budget for the Skagit Delta, a model of the distribution of sediment delivered to the delta, and an assessment of the importance of sediment to coastal community resilience, nearshore habitats, and valued marine resources. Results will inform restoration planning, adaptive management, andforecasts of ecosystem responses to fluvial sediment delivery.