The entrapment zo as long been considered an important region of the San Francisco estuary. It has been the subject of several previous studies, and its location has been suggested as an index of condition of the estuarine ecosystem.A close correlate of this location, X2 1, is now used as a management objective on the basis that X2 is correlated with the abundance or survival of several estuarine-dependent species. X2 is a crude tool to use for this purpose, but it can be refined only through improved understanding of the various mechanisms underlying these correlations. These mechanisms probably differ among species, but for species resident in and near the entrapment zone, the correlation with X2 may be due to variations in intenSity oftrapping mechanisms with pOSition of the entrapment zone. Therefore, it is worthwhile to investigate these trapping mechanisms, the nature of the entrapment zone as habitat, and the responses of the entrapment zone to changes in X2 or outflow from the Sacramento-San Joaquin Delta. Studies ofthe entrapment zone ofthe San Francisco estuary have been conducted in springs of 1994, 1995, and 1996. These studies have been funded by the Interagency Ecological Program and conducted by scientists from IEP member agencies, the Romberg Tiburon Center for Environmental Studies, Bodega Marine Laboratory, and the University of California at Santa Cruz. This report presents results from 1994, a low-flow year. Subsequent reports will describe results from 1995 and 1996 and highlight differences among years. This report is organized in chapters that comprise more-or-Iess independent papers, with an introductory chapter for the entire study. This Executive Summary presents highlights of the overall study plan and summarizes the major findings.