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Effects of a Removable Spillway Weir on the Behavior, Passage, and Survival of Juvenile Salmonids Noah S. Adams, John M. Plumb, and Dennis W. Rondorf Science Policy Exchange, September 12, 2007

We evaluated the behavior, passage, and survival of juvenile salmonids during tests of surface flow structures at Bonneville, McNary, and Lower Granite dams on the Snake and Columbia rivers. This presentation will focus on the results from the tests conducted at Lower Granite Dam. These results will be compared to results from tests of other bypass structures at Bonneville, McNary, and Ice Harbor dams.

Because juvenile steelhead migrate closer to the surface of the water than Chinook salmon, the performance of surface bypass structures when evaluated with juvenile steelhead is high. A conservative way to evaluate the performance of these structures is to measure their performance with Chinook salmon.

At Lower Granite Dam, survival of fish passing through the Removable Spillway Weir (RSW) was high and was not statistically different from survival of fish passing through the spillway. For example, in 2003, 2005, and 2006, survival of yearling Chinook salmon passing through the RSW was 98%(+2.3), 98%(+3.1), and 99%(+1.7), respectively. Survival of yearling Chinook salmon passing through the spillway during the same years was 93% (+6.0), 93%(+9.2), and 98%(+2.5), respectively.

The RSW increased the percentage of the fish passing through the spillway. In 2002 and 2003, the percent of yearling Chinook salmon that passed through the RSW and other spillways was 78% and 66%, respectively. During these same years when the RSW was turned off, the percent of yearling Chinook salmon that passed the spillway was 62% and 52%, respectively.

The effectiveness of the RSW was high. Effectiveness is the percent of the fish passing through the RSW relative to the percent of the water passing through the RSW. In 2002 and 2003 the effectiveness of the RSW for yearling Chinook salmon was 6.5:1 and 8.2:1, respectively. The effectiveness of the spillway without the RSW was 1.4:1 and 1.6:1, respectively. However, the RSW is always operated with some amount of flow through the spillway to improve tailrace egress conditions. This is often known as "training spill". When the "training spill" is taken into account, the effectiveness of the RSW declines to 2.6:1 in 2002 and 2.1:1 in 2003.

The RSW reduced forebay delay, especially when total river discharge was below 80 kcfs. Surface bypass structures tested at other mainstem dams showed similar results. In all cases, the effectiveness of the surface bypass structure was higher than the spillway effectiveness, survival through the surface bypass structure was equal to or higher than survival through the spillway, and forebay delay was reduced when a surface bypass structure was operating.

Related Sites:
Index of NW Council's Salmon Policy Exchange Presentations (Habitat, Mainstem, Estuary, Ocean Conditions)