The Sacramento-San Joaquin Delta is a 200,000 hectare freshwater marsh, formed principally by the mid to late Holocene rise in sea level. The marsh drains roughly 45% of the surface area of California and receives 50% of the state's annual runoff. Organic deposition within the marsh over the past 6000 years totals approximately 5 billion cubic meters, making this marsh one of the state's larger historic carbon reservoirs. During the late 1800s and early 1900s the entire marsh was reclaimed, involving construction of more than 2000 miles of levees. Today, those levees protect some of the most productive farms in the region as well as the water supply to more than 23 million Californians.   Microbial oxidation and compaction of the reclaimed organic soils of the Delta initiated widespread land subsidence. Many islands of the Delta lie more than 7 m below sea level. Continued oxidation, coupled with sea level rise is increasing the relative depth of these islands.  Anthropogenic accommodation space, or that space in the Delta that lies below sea level and is filled neither with sediment nor water, serves as a useful measure of the regional consequences of Delta subsidence and sea level rise. During the period 1900-2000, more than 2.5 billion cubic meters of anthropogenic accommodation space formed in the Delta. Based on modeling studies, by 2050 the Delta will contain more than 3 billion cubic meters of anthropogenic accommodation space. Subsidence and sea level rise also lead to a regional increase in the forces that can cause levee failure. Although failures take many forms, cumulative hydrostatic force on the levees provides a proxy for levee failure potential. These forces, coupled with anthropogenic accommodation space, increase significantly over the next 50 years, increasing the likelihood, and consequences of levee failure and island flooding. Additionally, there is a 2-in-3 chance that 100-year recurrence interval floods or earthquakes will cause catastrophic change in the Delta during the next 50 years. The collapse of the Sacramento-San Joaquin Delta will significantly disrupt critical water supply infrastructure for California along with major transportation routes, power supply lines, several ports and major cities. To date, there is no state or federal policy that addresses the consequences of, and potential responses to, gradual or punctuated landscape change in the Delta.