Juvenile Chinook Salmon Use of Reservoirs behind High-Head Dams in the Willamette River Basin, Oregon

Designing successful downstream passage structures at high-head dams in the Willamette River basin requires information on the size and distribution of juvenile Chinook salmon in the reservoirs.  We used rotary-screw traps to investigate the timing and size of Chinook salmon entering and exiting reservoirs.  Several gear-type were used to assess vertical and longitudinal (dam to head-of-reservoir) distribution of fish from spring through fall. The majority (>85%) of fish entering reservoirs were fry-sized subyearlings (35 mm mean fork length) in the spring.  Fish were distributed mainly in the upper end of the reservoir (near natal streams) and generally associated with shallow nearshore habitat in the spring.  As surface water temperatures increased in the early summer, fish moved into deeper water and returned to the surface in the fall when temperatures decreased.  Longitudinal distribution in the summer was bimodal with more fish near the head-of-reservoir and the dam forebay.  By fall, distribution was skewed with more fish in the forebay.  Mean subyearling size among reservoirs in the fall ranged from 134-208 mm, significantly larger than their counterparts rearing in streams above reservoirs.  The majority of fish exit reservoirs in the fall as subyearling parr.