Mono Lake Newsletter

Return ditch capacity frustrates restoration

by Heidi HopkinsMcBain & Trush, April 1998

Despite 1999 being a "normal" runoff year, Rush Creek received far lower flows than it should have. In fact, in three of the last four years, Rush Creek did not receive peak flows ordered by the State Water Board. Why? Because the return ditch that transports water to Rush Creek from below Grant Lake Reservoir still needs to be upgraded.

The Rush Creek return ditch is used by the L.A. Department of Water and Power (DWP) to deliver flows to Rush Creek when Grant Lake dam is not spilling—which is most of the time in most years. The ditch was originally constructed by DWP to get rid of any water that exceeded the capacity of the aqueduct—in effect, a safety valve for the aqueduct in wet years. The ditch receives water from the L.A. Aqueduct roughly a half mile down from the reservoir and transports it approximately one mile back to Rush Creek (see map). While the ditch originally carried 360 cfs, today only 160 cfs is allowed to flow through the ditch. DWP’s engineers concluded that if the ditch is to carry water year-round, flows higher than 160-cfs pose the risk of collapse in the event of an earthquake.

Continued use of the return ditch to put flows down Rush Creek was a compromise reached in the Water Board proceedings on Mono Basin restoration. The Mono Lake Committee along with others, such as the California Department of Fish and Game (DFG) and California Trout, sought to have DWP retrofit the Grant Lake dam to provide a direct outlet into Rush Creek. Instead—in part because of the multimillion-dollar cost of the retrofit—the Water Board ordered DWP to upgrade the return ditch to carry greater year-round flows (up to the 380 cfs capacity of the existing facilities) and ordered the stream monitoring team to evaluate the need for a Grant Lake outlet that would reliably provide flows needed for restoration of Rush Creek. The evaluation would be made at the end of eight to ten years of monitoring.

DWP and DFG are slowly chipping away at a set of issues that have hindered action to upgrade the return ditch as required by the Water Board in its 1998 restoration order. At issue is the Water Board’s requirement that there will be no "long-term loss of fish habitat in the stream." The DFG has concerns over how modifications will affect the existing fish habitat in the ditch, in which some of the largest brown trout in the system can be found.

All parties agreed this summer that conducting a pre-project habitat assessment was a critical first step towards implementing the Water Board order vis--vis the return ditch upgrades. DWP and DFG are now working out their agreement on what specific habitat qualities should be documented in the assessment. The Committee sees its role as being the "squeaky wheel" in the process, making sure that this matter doesn’t get buried on any one person’s desk. Things are moving slowly, but they’re moving.

Achieving Mandated Stream Restoration Flows

Even once seismic upgrades are completed, the upgraded Rush Creek return ditch capacity will still be inadequate to provide required Stream Restoration Flows (SRFs) in 40% of the years. As a result, the Water Board ordered that "Rush Creek augmentation" occur. In these years, up to 150 cfs will be diverted from Lee Vining Creek into Rush Creek. The timing of this procedure can be tricky, since the diversions must wait seven days after Lee Vining Creek peaks so as not to deprive Lee Vining Creek of its SRF, yet reach Rush Creek in time to augment its peak.

The monitoring team will evaluate whether the combination of Grant Lake reservoir spill plus flows diverted from Lee Vining Creek is a reliable means of providing the mandated SRFs for Rush Creek. They also will evaluate the effects on Lee Vining Creek and the need for construction of a new Grant Lake Reservoir outlet.

Heidi Hopkins is the Committee’s Eastern Sierra Policy Director. By the look of the pile of wood outside of her house she isn’t taking any chances with firewood this winter.

Return to Fall 1999 Newsletter

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