Culverts Disconnect Fish Streams
Half the culverts placed in creeks of Alberta's boreal forest hang in mid-air, preventing fish from travelling upstream. As a result, the hanging culverts break fish habitat into isolated sections. Fish cut-off in this way from the rest of streams are at higher risk of becoming locally extinct.
Researchers from the Alberta Conservation Association and University of Alberta found hanging culverts in all four watersheds they examined during 2002 and 2003. They inspected metal tubular culverts installed at stream crossings on roads built for forestry and the oil and gas industries.
The Swan watershed, north of Edmonton, stood out as having the highest proportion of impassable culverts, with 74% of its 122 industrial road culverts lying above water. The proportion of culverts found hanging in the other three watersheds varied from 27 to 42%.
Culverts were considered hanging if the lower end was suspended at least two centimetres above the water surface. This gap is considered enough to prevent small fish, like brook stickleback and lake chub, from swimming upstream. In fact, most of the hanging culverts were well over this threshold, resting 10 to 50 cm above streams.
The 3100 square kilometre Swan watershed has notable differences from the others that explain its high frequency of hanging culverts. The Swan roads are older, averaging 40 years, about twice the age of roads that researchers assessed elsewhere. As culverts age, the streambed below becomes scoured by water, increasing the possibility (by a factor of 1.065 a year in this region) that a properly-installed culvert no longer connects directly with water in the channel below.
Further exacerbating the likelihood of hanging culverts in Swan is its steeper stream gradients. Stream reaches at road crossings average a 3.4% rise over run, compared with 1.6 to 2.1% in the other watersheds.