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I ,) I I I 1\ I I I I II i I' f J I I & I: BIOLOGICAL OPINION Proposed Action occurring during the late fall and winter, except during flood releases. Variations in water depths are illustrated for representative stations in Appendix E of the BA. Flows arriving at Imperial Dam normally range from a high of about 14,400 cfs (which usually occurs in late spring to summer) to a low of about 2,500 cfs (which usually only occurs after a heavy local rainfall over the entire area below Imperial Dam, usually November or December, and when water entitlement holders are not taking water). Mexico's water order has to be delivered regardless of excess rainfall. Flows below Laguna Dam usually range between 300 and 500 cfs. Occasionally flows may range up to 4,000 cfs or higher if a heavy rainfall has occurred. Flows below Imperial Dam into the Colorado River Channel (California Sluiceway) normally range from about 250 cfs to about 350 cfs and are made up principally of return flows from the All-American desilting basins and gate leakage from the California sluiceway gates at Imperial Dam. Sluicing flows are released to remove sediment accumulated from the desilting basins in the sluiceway channel. These sluicing flows usually occur 2 to 3 times a month and consist of flows ranging from 8,000 to 10,000 cfs which are released for periods of about 20 minutes. These flows carry sediment to the Laguna Desilting Basin located about 2 miles downstream from Imperial Dam. Normal operational variances in elevation at Imperial Dam are from 180 feet to 180.9 feet msl. The top of the spillway at Imperial Dam is approximately 181 feet; this elevation is seldom exceeded. If water demand exceeds flows arriving at Imperial Dam for extended periods, the elevation behind Imperial Dam may, on very rare occasions (1 to 3 times per year), be drawn dow n to elevation 178.5 feet. Elevations are returned to normal within 3 days, the amount of time required for water to travel from Parker Dam. Elevations of the reservoir above Imperial Dam continuously fluctuate, to some degree, from day to day and during the day. Fluctuations are due to variability in the flows arriving at the dam and water entitlement holders' demand changes. Normal operational variances in elevation at Laguna Dam are from 138 feet to 151.3 feet msl. The top of the spillway at Laguna Dam is elevation 151.3 feet msl. The maximum elevation is met a few times a year when the storage is used to prevent or reduce over deliveries to Mexico. The lower elevations occur when it is necessary to use the water stored in Laguna Reservoir to meet Mexico I s water order. This normally occurs when water demand has increased after releases have already been made from Parker Dam and the water supply is reduced. Elevations behind Laguna Dam continuously change due to the changing water demand at Imperial Dam. Normal operational variances in elevation at Senator Wash reservoir are from 210 feet to 240 feet msl. The reservoir is currently on an elevation restriction at 240 feet msl for safety concerns regarding seepage above that level. Prior to the elevation restriction of 240 feet msl, the normal 25