Evaluation of a Surface Flow Bypass System for Steelhead Kelt Passage at Bonneville Dam, Washington

Abstract

A surface flow bypass system for juvenile Pacific salmon Oncorhynchus spp. began operation at the second powerhouse (B2) of Bonneville Dam on the lower Columbia River in spring 2004. This surface bypass, called the “B2 corner collector,” is the result of extensive modification of the original B2 sluice chute. Because steelhead O. mykiss are iteroparous, the effect of this bypass and the unmodified sluiceway at the older first powerhouse (B1) on downstream migration of postspawn steelhead (i.e., kelts) may be an important factor in the rate of iteroparity. As such, passage at Bonneville Dam (river kilometer (rkm) 234, measured from the mouth of the Columbia River) was examined to understand the efficiency of surface bypass in passing kelts. Steelhead kelts were collected, radio-tagged, and volitionally released from the juvenile bypass facilities at McNary Dam (rkm 465) and John Day Dam (rkm 345) on the Columbia River during spring 2004. Forebay residence times for kelts passing via B2 (with the corner collector operating) were significantly reduced relative to residence times published from a prior period with similar water flows at B2 (2002). Passage efficiency (nonturbine passage) at B2 significantly increased in relation to this same period. Over 80% of kelts at B2 and nearly the same percentage at B1 were routed away from turbines via surface flow routes passing up to 5% of total discharge at each powerhouse, indicating that relatively small amounts of surface flow are needed to pass kelts via nonturbine routes. Providing surface flow passage routes may provide an efficient means of bolstering iteroparity rates by increasing the number of kelts that successfully navigate Bonneville Dam during the spring.