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Nature 0r Nurture

Mother Nature doesn't need dredge spoils to restore a salt marsh on the old Cullinan hay ranch, a new federal study concludes. According to the study's U.C. Davis computer model, natural tidal flows from San Pablo Bay would create a better theoretical marsh on flat land than on land piled with sediment.

"On the models that did not utilize sediment, channels formed more quickly and the tidal marsh worked better," says Betsy Radtke of the San Pablo Bay National Wildlife Refuge, to which the Cullinan Ranch was added in 1991. The ranch, a former tidal wetland, has dikes that have caused soils to literally shrivel up and sink. If dikes were broken today, a brackish lake, not a marsh, would be produced. A century of oxidation has also produced acidic soils, unsuitable for growing vegetation favored by species such as the salt marsh harvest mouse and clapper rail, says Radtke. Spreading dredge spoils on the property has been offered as a means to raise soil levels and improve soil quality.

The computer model used five different configurations of internal levees, channels and entry points of water and added dredged material in one scenario. The results showed that within 5 years, natural flows from sediment-rich San Pablo Bay could create a marsh on 30% of the property. Within 10 years, the amount of marsh grew to 70%. Land covered with a thick layer of dredge spoils and opened to Napa River waters from the back of the ranch produced the suitable marsh elevations more quickly - in 3-5 years - but impeded development of tidal sloughs and created marsh that might not prove self-sustaining, according to Radtke.

The S.F. Bay Commission's Steve Goldbeck, whose job it is to find beneficial uses for dredge spoils, says the Cullinan study should have included a model combining sediment-lifts and San Pablo Bay flows and that the study never gave dredged material-facilitated restoration a fair chance. "It should not be construed that dredged material in general can't help," he says. "The situation is always site-specific."

Radtke agrees but says in the Cullinan model, the use of dredged material did not facilitate the marsh's development. "We're looking for the best biological option here, not just for the option that uses dredged material," she says.

Contact: Betsy Radtke (510)792-0222

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