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Subsidence in the Sacramento-San Joaquin Delta


Contents:

Introduction

Prior to Captain Pedro Fages visit to the Sacramento-San Joaquin Delta in 1772, the Delta was a tidal wetland that had been populated for at least 4,000 years by several Native American groups that inhabited the natural levees and sand dunes. Farmers settling in the Delta during the California Gold Rush of the 1850’s constructed levees to protect their crops and homes from flooding. A network of ditches later drained the soils and the island-surface elevations began to decrease due to subsidence of the peat soils.

Drainage of the Delta islands was essentially completed by the1930’s when it assumed its present configuration of about 65 island surrounded by 1,100 miles of man-made levees and over 700 miles of waterways.

Subsidence of the peat soils has caused the tidally influenced islands to become holes in which the land surfaces are now 10 to 25 feet below sea level. This large decrease in land-surface elevation has made the islands more susceptible to flooding. When most of the existing levees were constructed, the difference between the water level in the channel and the island surfaces was less than 5 feet. Because of the decreasing island-surface elevations, the levees are now required to hold back substantially more water than when they were originally constructed. The resulted increase in hydraulic pressures on levees that were constructed on foundations of sand, peat and organic sediments has caused about 35 levee failures since the 1930's. The primary reasons for levee failure are levee instability, seepage, and overtopping.

The cost of levee failures and flood damage is estimated to be in the hundreds of millions of dollars. Another important detrimental consequence of Delta island flooding is the movement of saline water into the Delta from Suisun Bay. This degradation of the water supply for two-thirds of California residents due to increasing salinity can result from the failure of one or more of the western Delta levees. If the flooded islands are not reclaimed the rate and area of fresh water and salt water mixing increase and evaporation losses increase, causing a long-term salinity increase. Even if the island is reclaimed, there can be substantial short-term increases in the salinity of the water supply.

There is increasing pressure to prevent Delta islands from flooding for protection of water quality, recreational use, agriculture, wildlife habitat, and property. As the islands continue to subside, levee repair and maintenance will become more critical and expensive. A critical factor in preventing further losses due to levee failure is stopping and reversing subsidence of the peat soils.

The deposition of the Delta peat soils began about 10 thousand years ago as marsh plants died, decayed and accumulated under oxygen-deficient conditions. Rising sea levels during the last 10 thousand years caused oxygen-deficient conditions to be maintained as decaying plants accumulated. The reclamation and drainage of these soils created oxygen-rich conditions in which the peat soils were consumed by microorganisms such as bacteria and fungi.

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Rates and Causes of Subsidence

There are over 750 million acres of peat soils worldwide. Subsidence rates vary substantially and depend mainly on climate and drainage. The oldest records of subsidence are for the polders in the western Netherlands, which were drained between the 9th and 13th centuries. These peat soils subsided 3 to 6 feet during 8 to 10 centuries (0.04 to 0.1 inch per year.) In contrast, the peat soils of the Sacramento-San Joaquin Delta have subsided at rates of 0.4 to 0.6 inches per year. The warmer California temperatures are the primary reason that the Delta soils subside at faster rates. Also, the water table in the Delta peat soils is kept at a deeper depth than in the Netherlands so that a larger mass of soil is exposed to oxygen thus, promoting greater decomposition by microorganisms. The warmer California temperatures cause the microorganisms to decompose the peat at faster rates.

Subsidence has been the highest on the central Delta islands where soil organic matter contents are the highest. Also, the Delta islands are often bowl shaped because more subsidence occurred in the relatively high-organic-matter peats in the center of the islands. The depth of the water table is usually deeper towards the center of the island where canals collect the island’s drainage water, which also contributes to higher oxidation rates.

The peat soil is a complex mass of carbon. Microorganisms use it as an energy source resulting in peat decomposition and release of carbon dioxide under the drained, oxygen-rich conditions. Studies by the USGS and the Department of Water Resources indicate that as much as 50 pounds of CO2 are released per acre per day from Delta peat soils. The amount of CO2 released is directly proportional to the amount of subsidence. Soil moisture and temperature influence the amount of CO2 loss. CO2 losses are greatest during the spring and summer and lowest during the winter. Wetter and colder conditions result in lower CO2 losses.

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Stopping and Reversing Subsidence: Wetter is Better

In 1991, the USGS and the California Department of Water Resources began a study of the effect of three different water management practices on carbon inputs and outputs and subsidence on Twitchell Island in the western Delta. The water management practices which are being considered in the plans for the development of wetlands on Twitchell and other Delta islands are (1) seasonal flooding from early fall through winter; (2) flooding from early fall through winter and irrigating twice during the summer and (3) permanent shallow (about 1 foot) flooding. The carbon inputs and outputs were measured for each site. Carbon is output as methane (CH4) and carbon dioxide (CO2). Carbon is input as decaying plant material.

The results of the study indicate that permanent flooding of peat soils can stop subsidence and increase the land-surface elevation. Because the wet conditions during the spring and summer prevent excessive loss of gaseous carbon, more carbon accumulates than is lost, similar to conditions when the peat soils were formed.

The carbon losses for the permanently flooded site were generally in the form of methane that forms from the decomposition of peat under oxygen deficient conditions. The carbon losses from this site were about 5 times lower than for the other two sites that were drained in the spring and summer months. Carbon lost from these sites was as CO2. The carbon losses for these sites were similar in form and amount to the losses from drained agricultural fields.

Current DWR and the USGS study objectives include quantifying the maximum peat accretion rate under permanently flooded conditions, and quantifying the subsidence rate of peat capped with sand in which the oxygen supply which drives the aerobic subsidence process is cut off from the peat.

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A Vision for the Future

A future vision for the Delta includes a return of some Delta islands to tidally influenced wetlands. This can be accomplished during decades of managed peat accumulation through the maintenance of flooded conditions. The increase in island surface elevations will decrease the need for levee maintenance and repair and contribute to the protection of water quality and supply, property, wildlife habitat, and recreational uses.