Opinion ID: 2632447
Heading Depth: 1
Heading Rank: 3

Heading: Cosumnes River: Impact on Salmon Migration

Text: The Cosumnes River lies south of the Well Field. The only remaining undammed river draining the Sierra Nevada's western slope, the Cosumnes supports steelhead trout and fall-run chinook salmon populations. The Draft EIR did not discuss the impact groundwater extraction at the Well Field would have on the river's flows and habitats. In public comments on the Draft EIR, however, several agencies, organizations and individuals expressed concern on the subject. The United States Fish and Wildlife Service noted that past groundwater withdrawals had significantly lowered groundwater levels in the area, which causes loss of flow in the Cosumnes River due to seepage through the riverbed and thus limits access of adult fall-run chinook to their spawning grounds. Any further withdrawals will almost certainly exacerbate this situation. The Fish and Wildlife Service comment urged an analysis of the potential effect of groundwater withdrawals on flow conditions in the river's spawning reach (between LaTrobe and Dillard Roads) and migratory reach (from the tidal zone to LaTrobe Road) during the fall and winter months. The National Marine Fisheries Service observed that the Cosumnes River is designated critical habitat for the Central Valley steelhead trout, a federally listed species, as well as habitat for a candidate species, fall/late fall-run chinook salmon. Further groundwater withdrawals in the area could reduce surface flow, significantly impacting recovery of listed and sensitive salmonid species. The Nature Conservancy, which manages the Cosumnes River Preserve (an area of 30,000 acres in which several state and federal agencies hold land interests), similarly observed that due to the lowering of the groundwater table the Cosumnes River now loses surface flow to groundwater, and, as a consequence, the river ceases flowing earlier in the year, stays dry longer into the Fall, and dries over an increasingly long reach, compared to historic conditions. Because water from fall rains must saturate an increasingly dry riverbed, significantly more fall water is now required for surface flows to reach the Delta and permit salmon migration; riparian habitats and seasonal wetlands are also adversely affected. Any increment of further lowering of groundwater will, in our view, have a significantly negative effect on these habitat and public trust values. Graham Fogg, a professor of hydrogeology at the University of California, Davis, who has studied the effects of groundwater extraction on the Cosumnes River, also warned that increased extraction could reduce stream flows, jeopardizing salmon migration. In particular, Fogg explained that while some reaches of the Cosumnes River are hydrologically disconnected from the aquifer in the region, modeling and field observations show a potential for connection upstream of Dillard Road and downstream of Highway 99. In response to these public comments, the FEIR states that available data suggest groundwater extraction at the proposed [W]ell [F]ield will not significantly impact flows in either Deer Creek [a tributary of the Cosumnes] or the Cosumnes River. The estimated impact on groundwater levels in the Cosumnes River area is less than five feet. Moreover, the deep aquifer from which the Well Field would draw is hydrologically disconnected from the Cosumnes River over most of its reach in the County. In the unconnected reaches, seepage from the river occurs whatever the regional groundwater elevation; further extraction would therefore have no effect on river flows. Hydrological connections exist upstream of Dillard Road and downstream of Twin Cities Road (about 7 miles downstream of Highway 99), but groundwater elevation changes in those reaches is expected to be no more than two feet and typically less than one foot. The FEIR concludes: The resulting impact on depletions from Deer Creek and the Cosumnes? River is not considered significant. Correspondingly, these depletions are expected to result in small but uncertain impacts on flows in Deer Creek and the Cosumnes River. The potential exception could be during periods of very low flow. During such periods of low flow, these depletions could change the timing and areal extent of the dewatering of the stream invert, potentially impacting aquatic and riparian-dependent species and habitat. The FEIR response also observed that the proposed extraction of 10,000 afa from the Well Field represented less than a 3 percent increase in the annual groundwater extraction underlying and adjacent to the Cosumnes River, and that agricultural wells located very close to the river and drawing from the region's shallower aquifer exert a much greater influence on local groundwater elevations and gradients than the proposed [W]ell [F]ield.