The San Francisco Bay/Sacramento-San Joaquin Delta system provides habitat for some 750 plant and animal species, provides drinking water for 22 million people, provides irrigation supplies for at least $27 billion in agriculture, and is a primary water source for California’s trillion–dollar economy. Both the ecosystems and the resource systems of this remarkable system, however, are in trouble. The Bay-Delta system is neither a healthy ecosystem nor a reliable source to meet growing demands for high-quality water. Recognizing the decline of this system and the urgency of its restoration, more than 20 State and Federal agencies have banded together to establish the multi-billon dollar, 30-yr CALFED Bay-Delta Restoration Program, which will develop and implement a comprehensive plan to accomplish four ambitious objectives:

1. Improve the reliability of water supplies in California,

2. Improve water quality in the Bay-Delta system,

3. Restore ecosystems within the Bay/Delta watershed, and

4. Stabilize Sacramento-San Joaquin Delta levees.

The success of the CALFED Program depends, among many other factors, upon the robustness of its plans and actions to the considerable buffeting that California’s highly variable climate will inevitably impose upon it. The Bay-Delta watershed spans the West Coast precipitation influences of the El Nino-Southern Oscillation climate variations, their interdecadal realization in the Pacific Decadal Oscillation, and other short- and (especially) long-term influences that are less well understood. In addition, recent observed winter-spring warming and streamflow-timing trends are either harbingers or analogs for future global warming effects in the State and threaten to unsettle crucial aspects of the State’s water supply system by adversely impacting the State’s snowpacks and snowmelt runoff. Recent studies of California’s paleoclimate also provide worrying evidence that the erratic precipitation regimes that have been observed (and largely accomodated) during the several hundred years of California’s development have been—by and large—benign and small in comparison to the precipitation variations over the past 1000 or more years. Thus, California’s climate has varied in ways that CALFED must be prepared to accommodate, on time scales from years to decades, as evidenced in its past and present climates as well as in projections of its future climate. These climate fluctuations need to be characterized (i.e., monitored, predicted, projected, or described probabilistically, depending on circumstance) to provide a basis for scientifically sound planning and management actions by the CALFED Program.