To examine the causes of land subsidence on marshes drained for agriculture, carbon fluxes and changes in land-surface elevation were determined on three islands in the Sacramento-San Joaquin Delta, California. Over the time period of March 1990 to May 1992, gaseous CO2 fluxes were determined approximately monthly using closed chambers, and dissolved carbon fluxes were determined from the dissolved carbon loads of drainage ditches adjacent to each field site. Surface elevation changes were measured continuously by measuring the distance between the land surface and an elevated structure anchored beneath the organic soil layer. Gaseous CO2 fluxes accounted for most of the permanent subsidence measured over the monitoring period. Gaseous CO2 fluxes are strongly affected by soil temperature. Net subsidence rates for the three islands, which have different depths of organic soils and water-management practices, range from 0.46 to 1.06 cm/yr. Estimates of dissolved organic carbon fluxes for all three islands were small relative to gaseous CO2 losses and represent <1% of the measured subsidence. ¿ American Geophysical Union 1996