Abstract
The stability of levees in the Sacramento-San Joaquin Delta is threatened by continued subsidence of Delta peat islands. Up to 6 meters of land-surface elevation has been lost in the 150 years since Delta marshes were leveed and drained, primarily from oxidation of peat soils. Flooding subsided peat islands halts peat oxidation by creating anoxic soils, but net accumulation of new material in restored wetlands is required to recover land-surface elevations. We investigated the subsidence reversal potential of two 3 hectare, permanently flooded, impounded wetlands re-established on a deeply subsided field on Twitchell Island. The shallower wetland (design water depth 25 cm) was almost completely colonized by dense emergent marsh vegetation within two years; whereas, the deeper wetland (design water depth 55 cm) which developed spatially variable depths as a result of heterogeneous colonization by emergent vegetation, still had some areas remaining as open water after nine years. Changes in land-surface elevation were quantified using repeated sedimentation-erosion table measurements. New material accumulating in the wetlands was sampled by coring. Land-surface elevations increased by an average of 4 cm/yr in both wetlands from 1997 to 2006; however, the rates at different sites in the wetlands ranged from -0.5 to +9.2 cm/yr. Open water areas of the deeper wetland without emergent vegetation had the lowest rates of land-surface elevation gain. The greatest rates occurred in areas of the deeper wetland most isolated from the river water inlets, with dense stands of emergent marsh vegetation (tules and cattails). Vegetated areas of the deeper wetland in the transition zones between open water and mature emergent stands had intermediate rates of land-surface gain, as did the entire shallower wetland. These results suggest that the dominant component contributing to land-surface elevation gain in these wetlands was accumulation of organic matter, rather than mineral sediment, and that accumulation of organic matter in emergent marshes is strongly affected by hydrologic factors. Re-established, non-tidal wetlands with managed hydrology can produce significant increases in land-surface elevations, which can help to improve levee stability and protect subsided islands from future flooding.
Highlights
The Sacramento–San Joaquin DeltaThe Sacramento–San Joaquin (SSJ) Delta was once a vast freshwater marsh, east of the San Francisco Bay, at the confluence of the Sacramento and San Joaquin rivers
Between December 1997 and March 2005, landsurface elevation as measured by the Sedimentation-erosion table (SET) in the west wetland increased by a median of 22.6 cm, which corresponds to a median rate of change of 3.1 cm/yr (IQR = 2.8 to 3.5 cm/yr) (Figure 3)
Between December 1997 and March 2005, landsurface elevation as measured by the SET in the east wetland increased by a median of 27.4 cm (IQR = 28.0 cm), which corresponds to an average rate of change of 3.8 cm/yr (IQR range = 1.4 to 5.2 cm/yr)
Summary
The Sacramento–San Joaquin (SSJ) Delta was once a vast freshwater marsh, east of the San Francisco Bay, at the confluence of the Sacramento and San Joaquin rivers. Peat deposits up to 18 meters thick formed in the SSJ Delta as accretion in the marshes kept pace with sea level rise during the last 6,000 years (Atwater and Belknap 1980; Shlemon and Begg 1975). The SSJ Delta currently consists of 57 islands and tracts protected from inundation by approximately 1700 kilometers of levees (Figure 1; DWR 1995). The islands have extensive networks of drainage ditches and pumps to remove water from the soils and return it to SSJ Delta channels in order to maintain crop-rootzone soils dry enough for agricultural use. Drainage of soils has resulted in landsurface subsidence of up to 7 meters below sea level (Figure 1)
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