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Soil Investigations for Data Collection in the Delta Initial Study/Proposed Mitigated Negative Declaration 88 1998, Moyle 2002). Because winter-run Chinook salmon use only the Sacramento River system for spawning, adults are likely to migrate upstream primarily along the western edge of the Delta through the Sacramento River corridor. Their migration past RBDD at river mile 242 begins in mid-December and continues into early August. The majority of the run passes RBDD between January and May, with the peak in mid-March (Hallock and Fisher 1985). The timing of migration may vary somewhat due to changes in river flows, dam operations, and water year type (Yoshiyama et al. 1998, Moyle 2002). Sacramento River winter-run Chinook salmon migrate into freshwater while still being immature and delay spawning for weeks or months upon reaching their spawning grounds (Healey 1991). Emigrating juvenile winter-run Chinook salmon pass the Red Bluff Diversion Dam beginning as early as mid-July, typically peaking in September, and can continue through March in dry years (Vogel and Marine 1991; NMFS 1997). Many juveniles apparently rear in the Sacramento River below Red Bluff Diversion Dam for several months before they reach the Delta (Williams 2006). From 1995 to 1999, all winter-run Chinook salmon outmigrating as fry passed the Red Bluff Diversion Dam by October, and all outmigrating presmolts and smolts passed the Red Bluff Diversion Dam by March (Martin et al. 2001). Juvenile winter-run Chinook salmon are present in the Delta primarily from November through early May based on data collected from trawls in the Sacramento River at West Sacramento (river mile 55) (USFWS 2006b), although the overall timing may extend from September to early May (NMFS 2012). The timing of migration varies somewhat because of changes in river flows, dam operations, seasonal water temperatures, and hydrologic conditions (water year type). Overall, designated in-water work windows would reduce exposure of sensitive fish species, including Sacramento River winter-run Chinook Salmon ESU and life stages to in-water work activities. The activities of the Proposed Project would be minor in scope and would not result in degradation of aquatic habitat or water quality conditions and any potential effects related to potential increase in suspended sediment concentrations and contaminants due to disturbance of the river bed would be negligible. Implementation of Mitigation Measures MM BIO- 14, along with MM HYD-1 and MM HAZ-1 through 4 would reduce potential impacts to Sacramento River winter-run Chinook salmon to: Less than Significant with Mitigation Incorporated. Sacramento Splittail (Pogonicthys macrolepidotus) The Sacramento splittail was listed as threatened under the federal Endangered Species Act (FESA) on February 8, 1999 (NMFS 1999). This ruling was challenged by two lawsuits (San Luis & Delta-Mendota Water Authority v. Anne Badgley et al. and State Water Contractors et al. v. Michael Spear et al.). On June 23, 2000, the Federal Eastern District Court of California found the ruling to be unlawful and on September 22 of the same year remanded the determination back to the U.S Fish and Wildlife Service (USFWS) for re-evaluation of their