Technical Report

Cochiti Reservoir Reregulation Interagency Biological Report

URL: https://webapps.usgs.gov/mrgescp/documents/Cochiti-Reservoir-Reregulation-Interagency-Biological-Report-1993.pdf

Date: 1993/06/30

Author(s): Allen C., Hanson B., Mullins C.

Publication: Interagency Biological Working Group. Contributions by: Mullins C., Hanson B., Pittenger J., Maracchini J., Johnson T., Pecos R., Pecos F., Herrera J., Allen C., Giesen L., Deragon W., Leutheuser R., Johnson L.

Abstract:

This report evaluates the biological effects of a proposal to use Cochiti Reservoir to reregulate irrigation water by occasionally and temporarily storing up to 5000 acre-feet of water during the summer irrigation season, from June through October of each year. Cochiti Reservoir is not presently authorized for storage of irrigation water - an authorization from Congress would be necessary to implement the reregulation proposal.

Reregulation may increase divertible water for the Middle Rio Grande Conservancy District (MRGCD) by capturing transit water from upstream storage not immediately needed for irrigation which currently must be passed through Cochiti Reservoir. From an ecological standpoint this passage of water is beneficial as it maintains the stability of the lake and provides water for plant and animal use in the riparian ecosystem downstream. Available information indicates a probable average savings to MRGCD of 440 acre-feet/year through this reregulation proposal, about 0.1% of the water diverted annually by MRGCD.

Data from the Otowi gage are analyzed to define the historic patterns of unregulated stream flow, especially flood events, in the Cochiti Reservoir reach of the Rio Grande. These data provide a template for defining an ecologically desirable pattern of management for Cochiti Reservoir. In particular, this analysis indicates that it is desirable to maintain stable pool levels during the summer. Historic flow data from the lower Rio Chama show that low flows on the Rio Chama are natural, and that winter low flows can be adequately supplemented without reregulation of Cochiti Reservoir by careful timing of water passages down the Rio Chama to replace evaporative losses in Cochiti's permanent pool.

An impressive delta and associated wetlands have developed in the headwaters of Cochiti Lake. Wetlands covered 199 acres in the delta area in 1991, with 47 acres of wetland inundated by the raising of the permanent pool in 1992. An additional 66 of the remaining 152 acres of wetland would be subject to flooding by the proposed reregulation storage. These lowest elevation wetlands are the most productive in the delta, as most organic sediments are deposited closest to the lake. The growing delta wetlands of Cochiti Lake are ecologically valuable due to the scarcity of such wetlands in the arid Southwest.

Even the relatively small amounts of water which would be held under the reregulation proposal would impose significant, negative impacts upon the vegetation and wildlife of the reservoir area, particularly in the headwaters delta region. The wetland vegetation of the delta would be directly impacted by reregulation water storage during the summer growing season. The fluctuating pool levels associated with reregulation storage would also tend to physically destabilize the channel and bars in the delta area, leading to less stable environments for littoral vegetation development. In addition, the increases in water utilization by MRGCD on which the proposed reregulation focuses would further dewater biologically sensitive downstream reaches of the Rio Grande.

Thus, the proposed reregulation is ecologically undesirable because of significant, direct and indirect, adverse biological impacts - it would move both Cochiti Reservoir and the downstream reaches of the Rio Grande farther away from the desired template of historic natural conditions and associated ecological integrity. Avoidance of these impacts is impossible as the impacts are inherent in the core reregulation proposal to store irrigation water during the growing season. Measures to mitigate these impacts would still involve overall diminishment of ecosystem integrity and wildlife habitat and would therefore be ineffective.

Therefore: The interagency biological working group recommends rejection of the reregulation proposal.

Significant, unrealized opportunities exist within the current authorization to greatly enhance management for fish, wildlife, and recreation at Cochiti Reservoir and still meet the primary flood and sediment control purposes of the dam. The interagency biological working group envisions a "desired future condition" for Cochiti Reservoir as a diverse, productive ecosystem occupying a strategic location on the Rio Grande flyway. We believe that the Cochiti delta area can develop into one of the most ecologically significant wetlands in New Mexico, with great benefits for local wildlife, migratory waterfowl, several threatened or endangered species, fisheries, and human enjoyment of these values. Thus, regardless of whether the reregulation proposal is pursued and implemented, the interagency biological working group recommends implementation of the following management measures for Cochiti Reservoir, all of which can be implemented within the current authorization:
1) avoid carryover storage;
2) maintain adequate flow capacity on the Rio Grande below Cochiti Dam to avoid carryover storage at Cochiti Reservoir;
3) restore vegetation that has been impacted by prolonged water storage;
4) use the improved annual operation scenario outlined in Figure 15 as a guide for operating Cochiti Reservoir;
5) all petitions for extraordinary water holding operations should be reviewed to insure consistency with the reservoir's authorization, including fish and wildlife; and
6) develop and maintain a single interagency biological team to enhance the ecological condition of Cochiti Lake and its delta.