Population increases in the Central Valley will result in significant land use changes, especially with respect to agriculture. It is estimated that nearly 800,000 acres of irrigated farmland will be converted to urban uses by 2040. For waterfowl, rice is by far the most important irrigated crop grown and loss of this habitat would have significant consequences for ducks and geese. However, a closer look at human population growth suggests that most of this growth will occur outside of today's rice growing regions. Of the nearly 500,000 acres of rice that is now grown in the Valley, less than 40,000 acres, or 8% of the total, is forecast to be lost by 2040.
The Sacramento and San Joaquin Valleys differ substantially in the amount and types of habitat provided for waterfowl. Riparian forests, semi-permanent tule marshes, and grasslands historically dominated the Sacramento Valley. Of the 1.5 million acres of wetlands that once existed in the Sacramento Valley, approximately 79,000 acres remain. Some 60% of the current wetlands are in private ownership and most are managed as duck clubs. Some of the most important areas of private duck clubs include the Willow Creek, Lurline, Butte Sinks, Colusa Trough, and District 10. Harvested rice fields provide winter foods and habitats for several species of waterbirds, especially if they are shallow flooded after harvest. Virtually all of the rice grown in the Central Valley occurs in the Sacramento Valley, with approximately 70% of this rice being flooded after harvest.
The San Joaquin Valley consists of the San Joaquin and Tulare drainage basins. Most of the San Joaquin Basin once consisted of seasonally flooded grasslands and vernal pools. Historically wetlands within the Tulare Basin were confined to Tulare, Kern, Goose, and Buena Vista lakes that covered some 625,000 acres. At present, there are 90,000 acres of wetland habitat in the San Joaquin Valley. Most of this habitat occurs in the Grasslands District, which is the largest contiguous block of wetlands in the Central Valley.
The vast watershed of the Central Valley drains into the Sacramento-San Joaquin Delta at the confluence of the Sacramento, San Joaquin, Mokelumne, and Cosumnes rivers. The inland network of 700,000 acres of sloughs and islands form an inverted delta that is one of the 60 largest river deltas in the world (Heitmeyer et al. 1989); only the Yukon-Kuskokwim and Copper River deltas of Alaska are larger on the west coast of North America. Because of its unique geologic formation, the Delta is the largest inland delta, and its waters flow through the largest brackish estuary on the Pacific Coast. Recent estimates indicate that as little as 7,500 acres of wetlands and 29,000 acres of winter-flooded agriculture may exist in the eastern Delta. Suisun Marsh currently contains 38,375 acres of managed wetlands that provide important food resources for migrating and wintering waterfowl. However, recent proposals to restore tidal flow to up to 6,750 acres of these managed habitats may reduce the carrying capacity of Suisun Marsh. As a result, DU and its partners in the Central Valley will determine the effects of tidal restoration on waterfowl populations in the Suisun Marsh.