Juvenile Salmonid survival, passage, and egress at McNary Dam during tests of temporary spillway weirs, 2009

Abstract

We evaluated behavior, passage, and survival of juvenile salmonids at McNary Dam in relation to the temporary spillway weirs (TSWs) using acoustic telemetry during 2009. The TSWs were located in spill bays 4 and 20 during spring and in spill bays 19 and 20 during summer. Our objectives were to assess the performance of the TSWs as a fish passage alternative. We also examined how tailrace conditions might have influenced fish survival by releasing drift buoys (drogues). The TSWs proved to be a relatively effective way to pass juvenile salmonids at McNary Dam (Summary Tables 1.1, 1.2, and 1.3), as was the case in 2007 and 2008. The TSWs passed about 14% of yearling Chinook salmon and 34% of juvenile steelhead with only 5-10% of total project discharge flowing through the TSWs. The TSWs and adjacent spill bays 16-18 passed 27% of subyearling Chinook salmon in the summer with 6-16% of total project discharge flowing through the TSWs. Based on the number of fish passing per the proportion of water flowing through the spillway (i.e., passage effectiveness), the TSWs were the most effective passage route. Passage effectiveness for fish passing through both TSW structures was 2.0 for yearling Chinook salmon, 5.2 for juvenile steelhead, and 2.7 subyearling Chinook salmon for TSW 20 alone. Higher passage of juvenile steelhead through the TSWs could have resulted from juvenile steelhead being more surface-oriented during migration (Plumb et al. 2004; Beeman et al. 2007; Beeman and Maule 2006). Based on passage performance and effectiveness metrics, TSW 4, located on the north end of the spillway, did not perform as well as TSW 20, located on the south end of the spillway. Passage proportions for TSW 4 were at least half that of the levels observed for TSW 20 for both yearling Chinook salmon and juvenile steelhead. This difference may be attributed to TSW location or other variables such as dam operations. Regardless of which TSW was used by fish passing the dam, survival through both TSWs was high (> 0.98 for paired-release dam survival) for yearling Chinook salmon and juvenile steelhead.