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Soil Investigations for Data Collection in the Delta Initial Study/Proposed Mitigated Negative Declaration 84 cycle is completed in the brackish and tidal freshwater reaches of the upper San Francisco Estuary. In addition, a freshwater resident life history type was found by Bush (2017), which primarily occurs in the Cache Slough region year-round (Sommer et al. 2011). Salinity requirements vary by life stage. Apart from spawning and egg-embryo development, the distribution and movements of all life stages are influenced by transport processes associated with water flows in the estuary, which also affect the quality and location of suitable open water habitat (Dege and Brown 2004; Feyrer et al. 2007; Nobriga et al. 2008). Delta Smelt are weakly anadromous and undergo a spawning migration from the low salinity zone (LSZ; 1–6 parts per thousand [ppt]) to freshwater in most years (Grimaldo et al. 2009; Sommer et al. 2011). Most of the later life-stage Delta smelt captured during the FMWT were collected in the 1 to 5 ppt salinity zone (Kimmerer et al. 2013). Spawning migrations occur between late December and late February, typically during “first flush” periods when inflow and turbidity increase on the Sacramento and San Joaquin Rivers (Grimaldo et al. 2009, Sommer et al. 2011). Adult smelt do not spawn immediately after migration to freshwater but appear to stage in upstream habitats (Sommer et al. 2011). Spawning primarily occurs during April through mid-May (Moyle 2002). There are a wide range of perspectives in the scientific literature regarding the extent to which the spatial distribution of Delta Smelt co-varies with X2 with more recent data and analyses suggesting factors other than X2 explain the distribution of the species (Murphy and Hamilton 2013; Manly et al. 2015; Latour 2016; Polanksy et al. 2018, Murphy and Weiland 2019). Dege and Brown (2004) found that larvae less than 20 mm rear 3–12 miles (5–20 km) upstream of X2 (Dege and Brown 2004; Sommer and Mejia 2013). As larvae grow and water temperatures increase in the Delta (~73°F [23 °C]), their distribution shifts towards the low salinity zone (Dege and Brown 2004; Nobriga et al. 2008). By fall, the centroid of Delta Smelt distribution is tightly coupled with X2 (Sommer et al. 2011; Sommer and Mejia 2013). While salinity is generally seen as a key driver of Delta smelt distribution, more recent research suggests other factors, such as water velocity (Bever et al. 2016), may be an important predictor of Delta smelt presence. Similarly, Murphy and Weiland (2019) demonstrate salinity alone may not be the best predictor of Delta smelt abundance and distribution. Overall, designated in-water work windows would reduce exposure of sensitive fish species, including Delta smelt, 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 Delta Smelt to: Less than Significant with Mitigation Incorporated. Steelhead – Central Valley DPS (Oncorhynchus mykiss irideus pop. 11)