Flood It And They Will Come

Some of the Delta's islands are slowly disappearing. A process called subsidence is transforming the rich peat soils that cover these islands into gaseous vapors. But an experiment to reverse the effects of subsidence has produced encouraging results. Researchers have begun "growing" vegetative material that will become peat, thus artificially reproducing the process that helped create the Delta islands.

"We just made history with this discovery," says Curt Schmutte, chief of the Department of Water Resources Flood Protection and Geographic Information Branch. "I knew it had to be happening, I just didn't know at what rate we could do it."

The subsidence research project began in 1992 when DWR and the United States Geological Society (USGS) built three 30-foot diameter test ponds on the south end of Twitchell Island. One pond remained permanently flooded, one was flooded only in winter, and one received seasonal flooding and summer irrigation. The researchers quickly observed that the permanently flooded pond produced a surprising buildup of dead plant material after only one year.

In the Beginning
At one time, the Delta was a 700,000-acre estuary of tule, reeds, bulrushes, cattails, and tidal marsh. During the last century, agricultural development and levee construction created about 100 islands. However today in some cases "island" is a misnomer, for some islands are really depressions or holes that dip 15 to 25 feet below sea level and are surrounded by levees. The surfaces of the central and western Delta islands are among the lowest. Farmers on these islands irrigate their crops by syphoning water over the protecting levees. The drainage water is then pumped back into the rivers and sloughs.

As subsidence increases, so does the risk of levee failure and island flooding. A serious levee break would flood homes, farms, and highways. Under certain conditions, it could draw salt water into the Delta from Suisun Marsh, jeopardizing water quality and water deliveries to millions of Californians. "If we continue to let the islands subside, it will be very difficult to sustain the levees," says Schmutte.

Disappearing Soil
At the heart of the problem is the soil. Many islands are covered with rich but unstable peat soil. On some islands the peat is (or was) more than 40 feet deep, and the islands are losing half an inch or more of soil a year. Although compaction and wind erosion contributed to the problem, most of the current subsidence is due to decomposition of the organic peat. The peat itself is largely partially decomposed plant material that oxidizes when exposed to the air.

Experts speculate that the Delta peat began to accumulate approximately 10,000 years ago after the last glaciation. As the polar ice caps melted, the sea levels rose. In the Delta, the rising sea levels created new marshes. Since saturated conditions slow the rate of plant decomposition, layer upon layer of dead plants piled up as the water levels continued to rise. The process continued over thousands of years, and the result was a thick layer of peat soil.

This is the process the research project is duplicating by artificially controlling water levels and nutrients to encourage plant growth. As plants die and create a layer of dead vegetation, the researchers raise the water level, encouraging new growth. Every cycle adds another layer of dead vegetation, which eventually should become peat.

"We are trying to make the plants grow at the maximum rate possible by regulating the water depth," says Schmutte. Cattails, which dominate the research ponds, prefer to grow in water about one foot deep.

"We have three to eight inches of well consolidated vegetative material that has grown in the last three years," he says.

"I'm not surprised that it happened, but I'm surprised at the amount," says Lauren Hastings, Soil Chemist with the USGS, the agency conducting the research and documenting its progress. Hastings is interested in the relationship between mat and peat: How many inches of dead vegetation will convert to how many inches of peat. It will likely take years to get that information, so the researchers are naturally cautious about their predictions. But they are optimistic enough to begin a larger demonstration. They are now attempting to replicate their earlier results by flooding a 20-acre Twitchell Island site. (Results from this study are expected in three to five years.)

"Large ponds will help us find the optimum point for growing peat," says Steve Deverel, a Delta hydrology and subsidence expert and independent contractor to DWR. "We want to watch the larger ponds for a couple of years. They are more representative of the critical areas in the Delta, although we will continue to study the smaller ponds too."

Deverel says they are optimistic but need more time to be certain it will work. "It will take a year or two. I think we'll have some pretty good estimates at that point as to how fast the material is going to accumulate," he says.

"It is very encouraging to get these results without much effort," says Schmutte. It bodes well for long-term restoration of the Delta islands. By growing peat, you are adding to the system by raising the soil level. One significant issue is that the land is committed to wetlands. You can't use the land for agriculture."

Schmutte adds that levees would still be necessary for flood protection of homes, highways, and other infrastructure. So peat growing is not a panacea and would most likely be used in Delta areas with the deepest peat soils and the highest risk of levee failures.

What is Next
Besides peat growing, there are related options under review. For example, blending peat with mineral material would reduce the percentage of organic material and therefore reduce the oxidation rate. This may allow land use for farming, hunting, and other activities. On the other hand, capping the peat soils with several feet of mineral material dredged from the rivers would reduce or prevent oxidation, control subsidence, and provide the added benefit of deepening the channels. Either option would stop or reduce the rate of subsidence, but neither would reverse the process, which is why the peat-growing project is generating such excitement.

"I think it (peat growing) will definitely work. It's just a matter of how long it will take," says Hastings.
"We can't just do it overnight. It would take a substantial period (many decades) to raise the islands up to the point they were in the 1850s. Further research will define the potential for raising islands," says Schmutte. "Our focus is to accelerate what nature has done on its own."

But it is telling that the researchers are willing to speculate. They sense they are working on what is at least a partial solution to a serious Delta problem. A process that, even with its limitations, will go a long way toward protecting the Delta from a disaster that could threaten the water supply for two-thirds of Californians.

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What is Subsidence?

The Delta developed its current form when levees were built and the "islands" drained. This reclamation and drainage exposed the nutrient-rich peat soils to the air, allowing them to be consumed by microorganisms (bacteria and fungi). This microbial oxidation results in peat decomposition and the release of carbon dioxide -- as much as 50 pounds per day per acre. There is a direct relationship between the amount of carbon dioxide released and the amount of subsidence. It is estimated that peat-soil oxidation has caused subsidence at rates of up to six inches a year, and parts of many islands have subsided more than 20 feet below sea level.