Small scale denil development for use in headwater streams in southwest Montana

Abstract

The Big Hole River is located in an agricultural valley in Southwest Montana and is home to the last fluvial (river dwelling) population of Arctic grayling (Thymallus arcticus) in the contiguous United States. Grayling mostly populate the tributary streams in the upper portion of the watershed, where there are many irrigation diversions, which greatly fragments grayling's natural habitat. While many of these irrigation diversions have fish ladders installed at them to assist with habitat reconnection, these ladder become impassable when the water levels get too low in the system or irrigators chose to block the fish ladders in order to divert more water. This study investigated and characterized a smaller scale Denil fish ladder that would use less water while providing adequate fish passage. Three different flow rate calculations were applied to a series of scaled Denils to compare to the expected flow rates of the full scale Denil to determine the scaled sizes to construct. A 0.6 scale and a 0.75 scale Denil were selected and hydraulic lab testing confirmed that 25.4 cm baffle spacing was the best for both scaled models. The fish swimming study, conducted at the outdoor flume at the Bozeman Fish Technology Center, used eight hatchery-raised grayling in each of the eight treatments. Each treatment was repeated 3 times using the 0.6-scale model for a total of 24 trials with 192 fish. Each treatment used a different combination of headwater depth (between 30.5 cm and 61.0 cm) and tailwater depth (between 15.2 cm and 61.0 cm). The grayling passed with near perfect success at all headwater and tailwater combinations except when the head difference between the headwater and tailwater was at its greatest (61.0 cm headwater and 15.2 cm tailwater). This preliminary study showed that grayling are willing to pass smaller-scale structures at a variety of flow rates but did not test a wide range of slopes, age classes or fish sizes. These results should be useful to water managers when looking to modify or install new Denil fishways in the Big Hole River Basin and around the western United States.