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 percent 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 percent.

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 percent rise over run, compared with 1.6 to 2.1 percent in the other watersheds.

Fragmented Streams

Suspended culverts fragment 20 percent of streams in the Swan watershed, isolating a total of 330 km of channel length on 90 streams. Altogether in the four watersheds, this study documented culverts cutting off 186 stream fragments and blocking over 1000 km of watercourses to emigrating fish. By isolating portions of forest streams, the hanging culverts considerably reduce the habitat accessible to fish.

Extrapolating the situation documented in these four watersheds to the entire 347,000 km² of boreal forest in Alberta presents a disturbing finding. Scientists estimate 7,600 to 10,700 hanging culverts sit under industrial roads. Those culverts cut-off fish access to somewhere between 36,000 to 50,000 kilometres of streams.

This estimate of the extent that culverts have transformed fish habitat is conservative because it doesn't include other ways in which culverts can stop fish passage. Culverts that get clogged by debris or that funnel water so it flows too fast for fish to swim in also add to habitat fragmentation.

Forestry and oil and gas development continue to spur road building throughout Alberta's boreal forests, and existing culverts continue to age. Unless road building and maintenance practices change, this study's authors warn, the problem of fragmented streams will only grow worse.


David Park, Michael Sullivan, Erin Bayne and Garry Scrimgeour. 2008. Landscape-level stream fragmentation caused by hanging culverts along roads in Alberta's boreal forest. Canadian Journal of Forest Research. 38(3): 566-575.

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