Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 6528
Title: Relative survival of juvenile salmon passing through the spillway of The Dalles Dam, 1997-2000
Author/Editor: R. F. Absolon, Earl M. Dawley, Benjamin P. Sandford, John W. Ferguson, Dean A. Brege
Publication Year: 2002
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Portland, Oregon
Contract Number: MIPR W66QKZ83437725
Pages: 67
Date: 12/01/2002

High rates of spill are presumed to increase passage survival for juvenile salmonid migrants.  However, at The Dalles Dam, high rates of spill may decrease spillway passage survival due to  1) a short stilling basin and shallow tailrace, which result in severe turbulence and lateral currents that may cause physical injury, and 2) a large proportion of spilled water moving through shallows and islands downstream, where rates of predation by gulls Larus spp. and northern pikeminnow Ptychocheilus oregonensi may be higher than than in the main river channel.

From 1997 to 2000, we conducted studies to evaluate the relative survival of juvenile Pacific salmon Oncorhynchus spp. through the spillway at The Dalles Dam.  Study fish were tagged with passive integrated transponder tags and released into the forebay of The Dalles Dam directly in front of the spillway.  Reference groups were released from a boat at the proposed site of the new bypass system outfall.  Detections of study fish were collected at Bonneville Dam, from the estuary pair trawl operated in the estuary, (~rkm 74), and from piscivorous bird colonies on Rice and East Sand Island (rkm 34 and 8, respectively).

Based on data collected through four years of study, we arrived at the following conclusions:

  1. Detection rates at Bonneville Dam for fish released through The Dalles spillway at 64% spill were significantly lower than for fish released downstream from The Dalles Dam.
  2. Based on data from the two years of direct comparison between spill rates, point estimates of relative survival for fish passing at 64% spill have been lower than for fish passing at 30% spill, but these differences have not been statistically significant.  However, when annual data from 1998 and 1999 were combined for analysis, the differences were significant for spring migrants (P = 0.039) in 1998, but not for summer migrants in 1999 (P = 0.19).  Respective point estimates for passage survival at 64 and 30% spill were 90.5 and 95.5% for spring migrants in 1998, 73.4 and 89.0% for summer migrants in 1998, and 95.9 and 99.5% for summer migrants in 1999.
  3. The combined data from 3 years of research (1997-1999) indicated that for summer migrants, spillway passage during daytime hours (adult spill patterns) produced lower passage survival estimates than spillway passage during nighttime hours (juvenile spill patterns), with respective daytime vs. nighttime estimates of 87.6 and 89.7% (P <0.01).  The combined data indicated that for spring migrants there was no difference between daytime and nighttime passage in 1997 and 1998 (89.8 vs. 88.8%, P <0.01) but there was a difference in 1999 (86.9 vs. 102.1%, P = 0.011).