Delta Subsidence in Northwest Washington

Lidar topography of active, un-urbanized, large deltas in northwest Washington shows a pattern of lower elevations in diked areas. In distal parts of the Skokomish, Nisqually, Snohomish, Stillaguamish and Nooksack deltas and in the active Skagit Bay sector of the Skagit delta, diked areas are about a meter lower than adjacent undiked areas. Undiked areas have fairly constant surface elevations, except for slightly higher natural levees along current major channels. Diked areas have higher elevations proximal to current and former channels and lower elevations farther from channels. In the Padilla and Samish sectors of the Skagit delta, where the delta front has probably been inactive for centuries (if not millennia) and there are no significant sediment sources, diked areas are not relatively lower. Elevations of the Duwamish and Puyallup deltas are dominated by filling for industrial development.

Subsidence in the Sacramento delta is commonly attributed to peat deflation. For the Mississippi delta, some authors have invoked tectonic causes. In northwest Washington, stable relative sea level in adjacent non-deltaic areas rules out tectonic subsidence and most delta sediments are not peaty. Variation of subsidence with position in the former channel network strongly suggests that compaction of silt- and clay-rich floodplain sediments is the primary cause of subsidence at rates as high as a meter per century. Uniform, higher elevations in undiked areas indicate that ongoing sedimentation during frequent floods is sufficient to compensate for subsidence.

To build and maintain dikes on these deltas is to trade the benefits of immediate flood protection and increased agricultural productivity for the costs of dike construction, lost fisheries resources, changes in wildlife habitat, lost carbon sequestration, and the ever-increasing hazard of a great flood that overtops the dikes.