Culvert Crossings as Potential Barriers to Fish Movement on the Kakwa River Watershed, Alberta
Tyler Johns and Thomas Ernst
The ability to travel throughout stream systems is extremely important for nearly all fish species. Obstructions to stream connectivity can limit the movement of fish to spawning or rearing habitats, and to locations where forage is optimal. Culvert crossings are commonly used in place of bridges for small to mid-sized streams but can be potential hazards to fish passage and sediment delivery when not maintained or installed correctly. The purpose of this project was to assess culvert crossings for their potential to impede fish passage, and to assess their potential for sediment delivery.
An evaluation of fish passage and sediment delivery of 75 culvert crossings in the Kakwa River watershed was conducted from June to July 2005. At each stream crossing, the condition of the inlet and outlet were visually inspected and classified as flush, perched, submerged or embedded. Culvert diameter, length, outfall drop height and plunge pool depth measurements were also recorded. At each stream crossing, eight elements were assessed to determine sediment delivery potential including right inlet ditchline, right outlet ditchline, left inlet ditchline, left outlet ditchline, outlet fill slope, inlet fill slope, right road surface and left road surface. Each of the eight elements consisted of up to seven erosion potential factors: 1) size of the sediment source area, 2) percent effective area of the sediment source, 3) percent of erosion control cover, 4) shape and size of the ditch, 5) soil texture, 6) slope of the source area, and 7) the level of road use. Each crossing was assigned a “score” that related to each element. A high score infers a significant erosion and sediment delivery potential.
Our assessments showed that 57% (n = 43) of culvert crossings represent potential barriers to fish passage as a result of hanging outlets. Sediment Delivery Index (SDI) assessments were performed on 50 culvert crossings. Of those, 72% were ranked as having a very high hazard score, suggesting significant erosion and sediment delivery potential. Given the high likelihood of culvert crossings resulting in potential fish passage barriers, it is necessary for responsible watershed stakeholders to monitor, maintain and replace culverts more frequently than other structures (i.e., open bottom arches or bridges) to provide and ensure fish passage over longer durations.