(between Whittier Narrows Dam and Coyote Creek) are fully concrete. The channel has mostly been constructed to withstand a 100-year flood, and reaches its maximum capacity just above Whittier Narrows at . Below the Whittier Narrows Dam the channel capacity is just ; most floodwaters are diverted to the Rio Hondo where the channel is much larger and deeper. The capacity of the San Gabriel River near the mouth is approximately . Water supply The San Gabriel River is an important source of water for the 35 incorporated cities and other communities in its watershed; despite the arid climate that requires water be imported from Northern California and the Colorado River, the San Gabriel still provides about a third of the water used locally. The Cogswell, San Gabriel and Morris dams are operated by the Los Angeles County Department of Public Works (LADPW) and can capture and store up to of rain and snow runoff. The Upper San Gabriel Valley Municipal Water District estimates that in an average year, between 95–99 percent of stormwater runoff from the San Gabriel River system is captured for storage, direct use or groundwater recharge. The California Department of Water Resources considered the San Gabriel River a "fully appropriated" stream, meaning that no new water rights may be taken. Two major groundwater basins or aquifers underlie the San Gabriel River watershed, separated by zones of impermeable bedrock and fault lines. Groundwater acts as the main long-term water storage of the San Gabriel River system, since the aquifers can hold many times more water than surface reservoirs. The San Gabriel Valley Basin covers a total of and has a storage capacity of 10.8 million acre feet (13.3 km3) of groundwater. The Central Basin is somewhat larger, with an area of and a storage capacity of 13.8 million acre feet (17.0 km3). Soil permeability, and thus natural groundwater recharge rates, is much higher in the San Gabriel Valley than in the Central Basin. Although both groundwater basins experience some overdraft, the deficit is more severe in the Central Basin. The LADPW operates an extensive series of spreading grounds which receive water from the San Gabriel River and allow it to percolate back into the regional aquifers. Due to the limited speed at which the ground can absorb water, the spreading grounds must be operated in tandem with surface reservoirs, which can capture big stormwater surges in winter