Session D3 - Fish Passage Restoration at the Briggsville Dam: Using Sediment Transport Analysis for Natural Channel Design

Agricultural diversion structures on South Boulder Creek (SBC; Boulder County, Colorado) have impaired fish movement for nearly 100 years. Although several of these structures have been modified for fish passage, their efficiencies remain unmeasured. Upstream fish movements were compared across two SBC structures and a control site in a 1-yr study. The upstream, channel-spanning structure had a ~7% slope, the downstream partial channel-spanning structure had a ~3% slope, and the control site was located at a small, channel-wide boulder vane. Five species of fish (Oncorhynchus mykiss, Salmo trutta, Rhinichthys cataractae, Catostomus catostomus,and C. commersoni) were marked with PIT tags, and their movements were monitored with paired antenna arrays at each of the three sites. Of the 1153 tagged fish, 663 were subsequently detected at one or more of the antennae. Upstream movement was lowest between November and March at all sites. More fish moved across the control site (n = 66) than across the low-slope (n = 31) or high-slope (n = 26) structures. More movement occurred across the control site in August and from April through June, a pattern also seen at the high-slope structure; movement over the low-slope structure was most prevalent in September and October, with a smaller pulse in April and May. Modifications to these structures have restored connectivity to a 2-km segment of SBC, but upstream fish movement rates remain lower across the structures than at the control site. Differences in upstream passage rates are related to structural design differences between the two fishways