Studies have shown that levee failure in the Sacramento-San Joaquin Delta can lead to salt water contamination of one of California9s most important surface sources of freshwater. This paper examines the seismic stability of one of the delta9s strategic levee systems for maintaining water quality: Twitchell Island9s Three Mile Slough levee. This levee will become unstable (factor of safety less than 1.00), due to low shear strength peat foundations beneath it, if the dead weight its suction-dredge placed sand toe berm is removed. The toe berm is highly susceptible to liquefaction from the Maximum Credible Earthquake (MCE) accelerations of several nearby faults based on the Simplified Liquefaction Procedure developed by Seed and others (1984), and field data presented in this paper. The Maximum Credible Earthquakes of several San Francisco Bay Area faults are capable of causing liquefaction of the toe berm and failure of the levee. A moderate earthquake of magnitude 6, centered within 14 mi of Twitchell Island, can also cause levee failure. Similarities between Twitchell Island and other delta islands suggest that the entire delta is vulnerable to even moderate earthquakes.