Adult spring Chinook migrate through the Upper Willamette from May to October. Spawning occurs <br />in tributaries upstream of the project area, with spawning activities peaking in mid-September (Oregon <br />Fish Division 2016). Pre-spawn mortality of adult UWR spring Chinook is highly variable between <br />years and has been shown to be dependent on water temperatures and origin of returning adults <br />(hatchery vs native). Pre-spawn mortality varies considerably but was consistently >80% of returning <br />females when composition of spawning fish was >80% hatchery origin and maximum temperatures <br />exceeded 20C (Bowerman et al. 2017). <br />° <br />Juvenile UWR Chinook salmon out-migrate through the project area as both sub-yearling and yearling <br />fish (Schroder et al. 2015). Juveniles that emigrate as yearlings are the most dominant life history type, <br />and these fish rear in the Willamette River year-round, including the river segment along the project <br />area. Peaks in juvenile out-migration typically occur during high flows in the spring, but sometimes <br />there is a very broad temporal distribution of outmigrants (Beamesderfer et al. 2011). <br />Table 3. Life history timing of UWR Spring Chinook Salmon in the vicinity of the project area. <br />SPECIES <br />JANFEBMARAPRMAYJUNEJULYAUGSEPTOCTNOVDEC <br />Spring Chinook <br /> <br />Juvenile Rearing <br />Peak Juvenile Migration <br /> <br />Adult migration <br />Shallow water habitats required by juvenile salmonids for migration and rearing are limited by the lack <br />of shallow water habitat complexity, absence of native and multi-layered riparian vegetation, and <br />substantial riverbank modifications. <br />HYDROLOGY <br />In general, the annual hydrograph for the Willamette River at the project area expresses seasonal high <br />flows between mid-November through January. On average, relatively moderate flows occur March <br />through mid-June and low flows occur through the summer and early autumn. <br />Data from an on-site USACE stream gage was used as the primary data source for the hydrologic <br />analysis. The gage (USACE EUGO) is located on the left bank water exchange structure of former <br />Steam Plant in Eugene, OR. Peak flows were also reported for the project area in the Lane County Flood <br />Insurance Study (FEMA 1999a) at the former USGS Gage 14158000 at State Hwy 125 near Springfield, <br />OR. Vegetation based (Non-regulatory) Ordinary High Water (OHW) marks were observed and <br />surveyed on April 26, 2018. These marks were then correlated to a discharge using the project-specific <br />hydraulic model created by Inter-Fluve. <br />The available post-dam construction annual peak flow record period (1976-2018) from the gage <br />(USACE EUGO) was used for the purpose of evaluating representative modern peak flow hydrology. <br />The peak flow evaluation followed the methods prescribed in the US National Flood Frequency <br />11 <br />