Road Ecology Center

Road crossings that utilize culverts on fish-bearing streams can impede fish passage in several ways. The most common impediments include large outlet drops, insufficient water depths, and excessive velocity. High velocities can act as passage barriers, especially for fish that migrate during high-flow periods of the year such as westslope cutthroat trout and Yellowstone cutthroat trout. We performed a basin-wide culvert study to investigate fish passage across a large basin in Montana. A second study (in progress) focused on the velocity component of fish passage. Our basin-wide culvert study was performed in the Clearwater River drainage near Seeley Lake, Montana. Fish species included westslope cutthroat trout, brook trout, brown trout, and bull trout. We studied 46 culverts over a range of culvert types and characteristics. We used a tiered approach to assess fish passage: analysis with FishXing, upstream and downstream population sampling, and direct-passage assessment. Results from the FishXing model from analysis of all 46 culverts indicate 76 to 85 percent are barriers at low flow, depending on the selection of minimum water depth. The upstream and downstream population-sampling analysis of a subset of 21 culverts indicated little or no significant difference in population characteristics (upstream characteristics compared to downstream characteristics). The direct-passage analysis of a subset of 12 culverts indicated no passage restriction at four culverts, some degree of passage restriction at seven culverts, and no passage at one. Our direct-passage study results may suggest more passage is occurring at low flows than the other methods suggest. The basin-wide study did not address passage issues during high flows. We have embarked on a second study (in progress) to assess this high flow passage with field sites at Mulherin creek, located near the north boundary of Yellowstone National Park. The site is an important spawning tributary for Yellowstone cutthroat trout and rainbow trout. We are using a combination of field studies and computational fluid dynamic (CFD) modeling to assess highvelocity fish passage over a range of flows. Field studies include fish monitoring and detailed velocity mapping using a traditional 1-D current meter and a 3-D acoustic Doppler velocimeter (ADV). We have chosen to monitor direct assessment of fish passage using Passive Integrated Transponder (PIT) tags in individual fish and fixed antennas placed at five culverts and throughout the system. Preliminary results indicate that inlet-velocity patterns can persist through the culvert barrel. Fish movement observations show use of the low-velocity region for passage even at high flows (average barrel velocities at the outlet up to 2.2 m/s) with passage restricted at times, even though areas of lower velocities exist.