California Delta peat soils can be “re-grown,” USGS finds

USGS scientist Robin Miller discusses the project on Twitchell Island

New wetlands reverse subsidence of Delta island, sequester carbon

Subsidence in the Sacramento-San Joaquin Delta of California – the steady loss of the region's fertile peat soils -- can be reversed by the creation of shallow, permanently flooded wetlands, the U.S. Geological Survey (USGS) has found.

In a pilot project on a deeply subsided island in the western Delta, USGS scientists recorded elevation gains of more than 10 inches from 1997 to 2005 as cattails, tules and other plants grew, died, decomposed and became new soil. The wetland plants also were successful at sequestering carbon– that is, taking carbon dioxide, a greenhouse gas, out of the atmosphere.

Subsidence of Delta islands -- many of which are lower than the surrounding waterways -- threatens the stability of the region’s levees, which in turn protect much of California’s water supplies. Water flowing through the Delta’s levee-protected channels goes to more than 20 million Californians and millions of acres of farmland in the Central Valley.

“Because the Delta’s peat soils developed in wetlands, re-establishing wetlands may be a way to accumulate new peat and raise land surfaces,” said Dr. Roger Fujii, Bay-Delta Program Chief for USGS’ California Water Science Center, which conducted the study. “Moreover, as a market for carbon credits develops, the very high rate of carbon sequestration could make growing these new wetlands an economically attractive alternative to current farming practices in the Delta.”

The study, launched in 1997, was jointly funded by the USGS Cooperative Water Program and the California Department of Water Resources.

USGS scientists established two seven-acre wetlands in the central part of Twitchell Island, where the land is up to 20 feet below sea level.The wetlands were constructed on a former agricultural field by excavating surface soil and using that soil as berms to contain the water. The east and west wetland sites were flooded and kept at about 22 inches and 10 inches deep, respectively, to see if rapid vegetation growth could be maintained over time.

On some of the deeply subsided Delta islands, the land surface falls up to an inch a year. But on Twitchell Island, as the wetlands plants grew, died and became compacted, scientists measured average increases in land-surface elevation of 1.2 to 1.6 inches a year between 1997 and 2005. The most productive spots in the deep wetland gained elevation almost twice as fast -- 2.2 inches a year.
 
“Re-established, non-tidal wetlands with a managed water supply can produce a significant increase in elevations,” Fujii said. “That could improve the stability of levees and begin to reverse the subsidence of the Delta’s peat islands.”

The study, “Subsidence reversal in a re-established wetland in the Sacramento-San Joaquin Delta,” by Miranda S. Fram, Gail Wheeler, Robin L. Miller and Fujii, has been approved by the USGS for submission to the online scientific journal San Francisco Estuary and Watershed Science.

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USGS provides science for a changing world.

The U.S. Geological Survey's California Water Science Center operates project offices in Sacramento and San Diego where more than 130 scientists bring a broad range of disciplines to modern water-management issues. The center also has nine field offices where scientists and technicians gather hydrologic data on California's surface-water and ground-water resources.