Assessment of Waterfall Fish Barriers in the Bow River Watershed, 2020-2021
To effectively safeguard against extirpation of native trout in Alberta, it is essential to protect native populations from hybridization and competition with invasive species. In Alberta, several subpopulations of native trout remain protected from invasive species primarily because of waterfalls (fish barriers) that impede upstream fish movement. Maintaining and isolating these subpopulations from invasion, in balance with the extirpation risks associated with isolation, is critical to the protection and persistence of native trout. Natural waterfall barriers in the Bow River headwaters are known to protect populations of westslope cutthroat trout (Oncorhynchus clarkii lewisi) from non-native rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis), and brown trout (Salmo trutta) invasions. Cataloguing waterfalls is a necessary first step to identify barriers that protect crucial westslope cutthroat trout populations, recognize opportunities to expand their range into secure habitat reaches above barriers, and prioritize future range expansion strategies to restore and reconnect existing populations. We used aerial imagery, GIS searches, the Government of Alberta barrier database, and backcountry hiking and tourism resources to compile a catalogue of potential barriers for assessment in the westslope cutthroat trout watersheds of the Bow River headwaters.
Between 2020 and 2021, we assessed 95 barriers in seven watersheds classed as Hydrologic Unit Code 8 of the Bow River headwaters with potential to provide upstream refuge from non-native fish species invasions. Six additional assessments were compiled into the dataset from previously measured barriers in the Bow River watershed in 2017–2018 for a total of 101 barrier assessments. Of these, 70 were considered impassible to local fish populations during the time of assessment, and 44 were considered absolute barriers based on flood inundation category. An additional 33 barriers were considered effective in terms of flood inundation. Knowing the locations and effectiveness of fish barriers in the Bow River headwaters is a key first step toward recognizing opportunities to expand the westslope cutthroat trout species range into secure habitat reaches, and to prioritize strategies to restore and reconnect existing populations.