You are hereMendota Dam, the Delta-Mendota Canal, and Mendota Pool

Mendota Dam, the Delta-Mendota Canal, and Mendota Pool


Mendota Dam

Mendota Dam (RM 204.8) is located at the confl uence of the San Joaquin River and Fresno Slough (Figure 5-1, Table 5-1). Fresno Slough connects the Kings River to the San Joaquin River, and delivers water to the south from Mendota Pool during the irrigation season, and delivers water to Mendota Pool and the San Joaquin River from the Kings River when the Kings River is fl ooding. Mendota Pool is the small reservoir created by Mendota Dam (3,000 acre-ft) and has a surface area
of approximately 1,200 acres. The pool behind the dam redistributes water delivered by the Delta- Mendota Canal to canals that convey water for agricultural use. Mendota Pool does not provide any appreciable fl ood storage. The water surface elevation in the pool is maintained by a set of manual gates and fl ashboards that are manually opened/removed in advance of high fl ow conditions. This process lowers the water level in the pool to pass high fl ows to reduce seepage impacts to adjacent
lands, but hinders distribution of fl ows into the canals.

Mendota Dam serves as a complex water distribution manifold to many diversions and riparian pumps, all of which are unscreened or do not meet National Marine Fisheries Service (NMFS) and California Department of Fish and Game (CDFG) screening criteria for salmonids. This complex area of water diversions will be a considerable constrain to salmonid restoration efforts due to the unscreened diversions and large volume of water exchanged in the Mendota Pool. Mendota Dam and Mendota Pool have been used for irrigation diversions since the late 1800s, and had historically depended on San Joaquin River and Fresno Slough fl ows to divert into irrigation canals originatingfrom Mendota Pool. After completion of Friant Dam in 1948, fl ows to Mendota Pool from the San Joaquin River was greatly decreased. Completion of the Delta-Mendota Canal in 1951 delivered water pumped from the Bay-Delta to Mendota Pool. The DMC has a rated capacity of 4,600 cfs (DOI, 1981); however, typical water delivery by the DMC during the irrigation season is approximately 2,500 to 2,800 cfs (Figure 5-4), with no water delivered to Mendota Pool by the San Joaquin River or Fresno Slough during the irrigation season. Five diversion canals extract all but 500 to 600 cfs of
water delivered to the Mendota Pool complex by the DMC. Mendota Dam releases this remaining fl ow into Reach 3 of the San Joaquin River. This release fl ows approximately 22 miles downstream to Sack Dam, where it is diverted into the Arroyo Canal.

Although Mendota Dam is much smaller than Friant Dam, it is substantial barrier to the upstream and downstream migration of salmonids. While there is a fi sh ladder on the dam, it has been inoperable since the late 1940’s, and erosion on the downstream side of the dam has perched the entrance to the ladder above the water surface. Therefore, adult salmonids (and other fi sh) cannot migrate upstream past the dam during typical fl ow conditions (it is potentially passable when all the boards
are pulled, but water velocities may still be too great for passage) and the fi sh ladder would need to be reconstructed to be usable. In addition, downstream migrating juvenile fi sh would likely incur high entrainment losses through the unscreened diversions and canals.

 

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