Impacts of Water Diversions
on the Mono Basin
When the Los Angeles Department of Water and Power began diverting Mono Lake's tributary streams in 1941, the lake no longer received its total annual freshwater inflow. By 1982, Mono Lake dropped 45 vertical feet, lost half its volume, and doubled in salinity. Vast alkali dust flats were exposed, the islands where birds nest became connected to the mainland, and the brine shrimp and alkali flies on which millions of migratory birds depend were threatened.
The drop in lake level depleted lake-fringing wetlands, layers of freshwater around stream mouths, and brackish lagoons—all of which were important habitat for waterfowl. Ultimately, ducks and geese arrived in numbers less than 1% that of pre-diversion times.
MONO BASIN STREAMS
Water diversions by DWP dried up the streams and destroyed the stream ecosystems, including the fisheries. The dessication of the streamside forests left the stream banks highly vulnerable to erosion. In years when snowmelt runoff was greater than the diversion and storage capacity of DWP's aqueduct system, DWP released large sudden flood flows down the creeks.These flood flows, in conjunction with the lowering of Mono Lake, "resulted in major incision of tributary deltas and groundwater tables and left former side channels stranded above the newly incised main stream channels" (D-1631 p.88). Thousands oftons of floodplain sediments were washed into Mono Lake. The erosion resulted in a "shifting, widening and straightening of the primary channels... reduced flows and the widening of the channels eliminated overbank flooding which, in turn, reduced the vigor of riparian vegetation and wetlands." (D-1631 p.88).
"The loss of the riparian community had serious impacts on the fishery of Rush and Lee Vining Creeks. Based on the data in the Draft EIR, a total of 204.4 acres of mature riparian vegetation had been lost on the four streams by 1989. Losses of over 100 acres of meadow and wetland acreage had also occurred" (D-1631 p.88).
The adverse impacts of these flood events were in direct contrast to the effects of flood flows prior to diversions. Formerly, the well-vegetated floodplain surface would trap sediments when flood waters flowed over channel banks, spreading seeds, moisture and bank-building soils. The riparian systems had been built and maintained by flooding.