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Subsidence and Carbon Fluxes in the Sacramento/San Joaquin Delta

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Subsidence and Carbon Fluxes in the Sacramento/San Joaquin Delta U.S. Department of the Interior Subsidence and Carbon Fluxes in Subsidence U.S. Geological Survey the Sacramento/San Joaquin Delta, Fact Sheet California INTRODUCTION The Sacramento/San Joaquin Delta was once tidal marshland and was formed by the confluence of the Sacramento and the San Joaquin Rivers. The delta covers more than 1,000 square miles and is bor­ dered by the major population centers of the San Francisco Bay area, Sacramento, and Stockton (fig. 1). The delta took its current form by the 1930's when drainage of 100 islands and tracts and construction of 2,250 miles of levees were completed. The land surface on the delta islands has progressively subsided below sea lev­ el since the early 20th century. The integ­ rity of the levee network is threatened be­ cause subsidence increases the differences in water levels maintained by the levees, which can cause levee failure and conse­ quent flooding of the delta islands. Levee failure affects the quality and beneficial use of water in the delta, which is the major source of California' s water supply to millions of residents and acres of agri- 38°00' cultural land. This Fact Sheet briefly sum- marizes the research done by the U.S. Geological Survey, in cooperation with the California Department of Water Re­ sources, to assess the causes of subsi­ dence, processes that affect subsidence, and carbon fluxes in the delta. 10 MILES STUDY OBJECTIVE AND 10 KILOMETER S APPROACH Results of the initial phase of the study demonstrated the small influence of withdrawals of ground water and natural gas on subsidence on the delta islands. Subsequent investigations were directed toward understanding variations of subsi­ dence in space and time by assessing the historical leveling surveys and land sur­ face elevation changes relative to expo­ sure of electrical transmission tower foun­ Figure 1. Location of study area and sites. dations and by evaluating land-surface el­ evation changes in relation to carbon flux­ es and changing water-management prac­ ongoing on Sherman Island, Jersey Island, RELATION OF SUBSIDENCE tices. Subsidence data were obtained from and Orwood Tract. Land-surface elevation TO ORGANIC-MATTER CON­ the historical leveling surveys done on Ba­ changes and carbon fluxes also are being TENT AND CARBON FLUXES con Island, Mildred Island, Lower Jones evaluated for different flooding regimes as Tract, and Sherman Island. The evaluation part of a proposed conversion of Twitchell Evaluation of historical subsidence of carbon fluxes relative to subsidence is Island wildlife habitat. data indicates that subsidence is slowing 100 10 l) spring to early autumn. Increases in land • measurement site 01 (/) <fl UJ • :::E 0 measurement stte 02 surface elevation during the winter :c a:< e measurement site 03 () • (!) o measurement sHe 04 months are due to rising ground-water lev­ ~ • ;?; D. extensometer site • x ~ ::> els because the peat soils are buoyant. The u.i •' ri () w net land-surface elevation loss of about z • • 0 UJ • § t:l 0.3 in/yr, which occurred during the length 1i3 9 IIl .. gg: of record, generally agrees with the sub­ :::> a:z (/) t'jg sidence calculated from the loss of C02 u. a: 0 < •• 0 (fig. 4) . zf­ :::> •• 0 • ::E <( 20 STUDY FINDINGS AND 0 5 10 15 20 25 30 35 40 45 50 Figure 3. Carbon-dioxide fluxes measured WATER MANAGEMENT PERCENTAGE OF at Orwood Tract sites, April 1990 to June ORGANIC-MADER CONTENT OF SOIL 1992. CHANGES Water- and land-management prac­ tices and environmental factors influence Figure 2. Relation of subsidence from sites within a 20-acre field on Orwood carbon mass balances and determine 191 o to 1988 to organic-matter content of Tract is shown in figure 3. Temperature whether land surfaces of the delta islands soil on Sherman Island. effects account for the high fluxes from will accrete or subside. Water- and land­ late spring to and early autumn. management practices include irrigation over time and that areal variability of sub­ and drainage and other agricultural prac­ sidence rates is related to the organic mat­ Differences in moisture content and tices. Environmental factors include the ter in the soil. Subsidence at the electrical spatial variability in the organic-matter chemical composition of waters that flood transmission tower foundations from 1910 content and thickness of the peat account the islands, water temperature, and the to 1988 correlates with organic-matter for most of the variability in C02 fluxes thickness and characteristics of the peat. content of soil on Sherman Island (fig. 2); among the sites. C02 fluxes are normally highest under low moisture contents. Be­ the relation explains about 70 percent of Preliminary findings on Twitchell Is­ cause thicker peats have more carbon the areal variance in the amount of land land indicate that constant flooding sub­ available for microorganisms to utilize, subsidence during this 78-year period. stantially lowers carbon fluxes relative to more C0 is released. The peat at the ex­ 2 either intermittent flooding or constant tensometer site on the Orwood Tract is drainage. During a 6-month period, the Land surfaces in the central delta thicker than that at the other sites and usu­ amount of carbon that left the flooded site (Mildred and Bacon Islands and Lower ally drier; therefore, C0 fluxes are usual­ 2 was three to five times less than the Jones Tract) have subsided an average of ly the highest at that site. More than 30 amount that left three sites that were either about 3 inches per year (in/yr) since 1921. pounds of carbon per day in the form of drained constantly or intermittently flood­ Sherman Island in the western delta has C0 flows out from an acre of drained peat 2 ed. The differences in the amounts of car­ subsided less than 1 in/yr since 1910. The soil in the delta. higher subsidence rates on the central . bon that left the other sites were not signi­ ficantly different. A net accumulation of delta islands are the result of more orgamc Subsidence was measured with an ex­ carbon also was detected on the flooded is­ matter in the soils than on western delta tensometer and calculated from C0 - flux 2 land. These preliminary findings indicate islands. Sherman Island and other western measurements. The extensometer mea­ that subsidence can be stopped by perma­ delta islands have been subject to more sures changes in land-surface elevation of nent flooding. Planned flooding will sub­ inundation and sediment deposition from the peat layer relative to the underlying stantially change the carbon budget _by_ the Sacramento and the San Joaquin mineral layer. Decreases in land-surface decreasing the amount of oxygen within Rivers, which result in less organic matter elevation are due to carbon loss (oxidation the organic soils and probably will result in the soils and lower subsidence rates of peat) and declining water levels from relative to the central delta islands. The in decreased subsidence rates. organic-matter content of surface soils in the central delta is about 50 percent com- ffl e Land-surlace eleva1ion pared with about 20 to 30 percent for the ~ based on C0 fluxes 2 For more information on studies of western delta. ;?; z land subsidence in California please Q .80 '< write: Results of ongoing research indiCate Measured elevation ~ / that subsidence in the delta is caused pri­ ~ .60 ~ a: marily by the oxidation of peat to carbon ::> If) District Chief 6 .40 dioxide (C02). The peat is a complex mass of carbon; microorganisms utilize it as an ~ U.S. Geological Survey .30 A M J J A S 0 N 0 J F M A M J J A S 0 N 0 J F M J J energy source and release C02. The C02 1990 1991 1992 Water Resources Division fluxes measured on Jersey and Sherman Federal Building, Room W-2234 Islands and on Orwood Tract since April Figure 4. Land-surface elevation measured 2800 Cottage Way 1990 are related primarily to soil tempera­ and calculated from carbon-dioxide fluxes ture. For example, the C02 flux at five on the Orwood Tract. Sacramento, California 95825 December 1994 FS 94-049 .
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