LAND-SURFACE SUBSIDENCE FROM PEAT OXIDATION AND COMPACTION IN THE SACRAMENTO-SAN JOAQUIN RIVER DELTA, CALIFORNIA

Much of the Sacramento-San Joaquin Delta of California (hereafter, the Delta) was reclaimed for agriculture 100–150 years ago, and since that time, local land surfaces have subsided by as much as 8 m below sea level. Land-surface subsidence in the Delta is primarily a result of oxidation and compaction of Holocene peat subsequent to drainage. As the Delta continues to subside, flood hazards increase due to increased hydraulic head on man-made levees. Although the mechanisms of land-surface subsidence are well known, the depth to which physical and chemical processes impact the peat have yet to be fully examined. Such information is vital for understanding how to mitigate future land-surface subsidence.

During the summer of 2005, we collected peat cores on eight islands in the Delta using a modified Livingstone corer. The sites included four sets of both reclaimed, farmed islands and relatively undisturbed, marsh islands situated in channels adjacent to the farmed islands. This afforded a pre- and post-reclamation comparison of the peat. Laboratory analyses of the cores were conducted for bulk density, percent organic matter, and radiocarbon (for age control of the peat). The thickness of the peat on the marsh islands ranged from 4.3 to 7.8 m. The total peat thickness at the center of the farmed islands varied from 1.6 to 2.4 m, and included a compacted and oxidized zone above relatively undisturbed peat that ranged from 0.4 to 0.8 m in thickness. Mean bulk density of marsh island peat (0.16 g/cm³) was lower than mean bulk density of relatively undisturbed farmed island peat (0.22 g/cm³) (Student's t-test, p < 0.0001), but was significantly less than the mean bulk density of compacted and oxidized peat (0.72 g/cm³) found at the surface of the farmed islands (Student's t-test, p < 0.0001). As bulk densities increased in the cores, percent organic matter content decreased. Higher bulk densities and lower organic content of peat on reclaimed farmed islands relative to marsh-island sites are evidence of compaction and oxidation due to reclamation and agricultural practices. The thickness of the relatively undisturbed peat that remains on the farmed islands ranged from 1.1 to 1.6 m, illustrating that little of the original peat column has been left untouched by the impacts of drainage and reclamation.