Stream Crossing Inventories in the Swan and Notikewin River Basins of Northwest Alberta: Resolution at the Watershed Scale


J. Tchir, P. Hvenegaard, G. Scrimgeour


Aquatic habitat fragmentation, degradation and encroachment resulting from industrial
activities can alter the distribution, abundance and subsequent viability of stream fi sh
populations. Using GIS tools and fi eld assessments we documented crossings of streams by
road networks and assessed their potential to fragment stream habitats and to act as sources
of sediment intrusions into stream channels in two boreal watersheds in Alberta, Canada. Our
data showed that the density of stream crossings in the Swan River watershed (0.24 crossings
/ linear stream km) was about three times higher than that in the Notikewin River watershed
(0.068 crossings / linear stream km). Culverts were the predominant crossing structure in
both basins and occurred predominantly on fi rst to third order streams. Our fi eld assessments
suggest that the majority of culvert crossings in the Notikewin (61% of all crossings assessed)
and Swan basins (74%) have the potential to fragment stream habitats. Culverts can obstruct
fi sh passage through various processes. Undersized or improperly graded culverts can result
in velocity barriers and cause downstream scouring, resulting in a perched culvert. Assuming
that these structures impede fi sh passage, culvert barriers could result in limited access to about
20% and 9.5% of the total length of stream habitats in the Swan and Notikewin watersheds,
respectively. Assessments also identifi ed many culverts less than bankfull width that reduce
the surface area of benthic habitats. From our qualitative assessments relatively few culverts in
the Notikewin (17%) and Swan River watershed (18%) potentially contributed moderate levels
of silt to stream channels. The propensity of culverts and bridges to potentially contribute
high amounts of sediment to stream channels in the Swan (19% of all culverts; 36% of all bridges) was higher than that in the Notikewin River watershed (3% of all culverts; 8% of all bridges). Taken
together, our preliminary assessments suggest that fragmentation of stream habitats and sedimentation related to development of road networks is an important management issue and that additional efforts are required to better understand the effects of stream crossings of stream fish communities in northern boreal systems.

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