The North Delta Flood Control and Ecosystem Restoration Project would address the need for flood control solutions that are integrated with ecosystem improvements. The existing and historic conditions that warrant flood control and ecosystem quality improvements are described below.
The Mokelumne and Cosumnes Rivers and the Morrison Creek stream group do not currently have sufficient channel capacity to safely convey 100-year peak flows from Sierra Nevada watersheds through the North Delta to the San Joaquin River. Current channel capacities for the North and South Forks of the Mokelumne River are approximately 40,000 cubic feet per second (cfs); the combined channel capacity required to safely convey flows from a 100-year flood event would be 90,000 cfs. The lack of channel capacity, combined with other constrictions in vulnerable areas (e.g., displaced boats lodged against bridge abutments), make a number of areas in the North Delta vulnerable to flooding. Areas that have flooded in the past include the Point Pleasant area, McCormack-Williamson Tract, Dead Horse Island, Staten Island and New Hope Tract, Brack and Canal Ranch Tracts, and the Franklin Pond area. The potential for flooding also threatens important public facilities and institutions in the North Delta area, including Interstate 5, the Union Pacific Railroad line, and the Rio Cosumnes Correctional Center.
The North Delta is also susceptible to levee failure during peak flows. Levees on McCormack-Williamson Tract and Dead Horse Island have frequently been overtopped during large floods, and many other levees have been subject to structural failure during past storm events. Failure of Bay-Delta levees can:
- result in flooding of Delta communities, farmland, habitat and key roads and highways;
- expose adjacent islands to increased wave action, increased seepage, and possible levee erosion;
- affect water supply distribution systems; and
- affect flow patterns, potentially resulting in adverse impacts to water quality, if levee breach is not repaired.
Degradation and the loss of habitats that support various life stages of aquatic and terrestrial biota are a primary concern in the North Delta. These habitat changes come from many causes such as reclamation,dredging, upstream development, water diversions, introduction of exotic species, and water quality impacts. Many of the seasonally inundated lands in the Bay-Delta system that historically provided habitat to a variety of bird and animal species have been converted to agricultural, industrial, and urban uses. Levees constructed to protect lands in the Delta from inundation eliminated fish access to shallow overflow areas, and dredging to construct levees eliminated the tule bed habitat along the river channels. Upstream water development and use, depletion of natural flows by local diverters, and the export of water from the Bay-Delta system have altered hydrodynamic processes. This has resulted in changed seasonal patterns of inflow, reduced Delta outflow, and diminished the natural variability of flows into and through the Bay-Delta system. In addition, many exotic species that are adapted to the changed Delta conditions (warmer water temperatures, lower salinities, less seasonal variability, for example) have invaded the estuary and predate or displace native Delta species. As well, water quality impacts have significantly degraded habitats supporting aquatic and terrestrial biota of concern in the North Delta.