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.