CHANGING WATER-BALANCE OVER TIME IN RUSH CREEK, EASTERN CALIFORNIA, 1860-1992

Rush Creek, the principal tributary to Mono Lake, has undergone profou nd hydrologic modifications as a result of flow regulation for hydroel ectric generation and irrigation, diversions for irrigated agriculture , and diversions for water export to the City of Los Angeles. Lower Ru sh Creek (the lowermost 13 km down-stream of Grant Lake Reservoir) was dry by 1970, but now receives flow as a result of court-ordered effor ts to restore former ecological conditions. Using available historic d ata and recent field measurements, we constructed the water balance fo r Lower Rush Creek, identifying six distinct historical periods charac terized by very different patterns of gain and loss. The hydrologic pa tterns must be understood as a basis for modeling ecosystem response t o stream-flow alteration. A gradually gaining stream under natural con ditions, the advent of irrigation diversions caused the middle reaches of Lower Rush Creek to be often completely dry, while irrigation-rech arged springs still maintained a baseflow in the downstream ''Meadows' ' ranch. Increased water exports from the basin subsequently reduced i rrigation and dried up the springs.