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  • Scott Tansowny

Alberta's Eastern Slopes Streams, an Endangered Habitat? Part 1: Habitat Fragmentation

The Eastern Slopes of Alberta’s Rocky Mountains are an amazing place. Little blue lines of cold, clear water cut their way through rocky canyons, drop down sheer cliffs, flow around massive boulders, and meander through meadows and muskeg. These beautiful streams of the Eastern Slopes are inhabited by many creatures including our native salmonids, fish from the family Salmonidae — our trout, cutthroat and rainbow; our char, bull trout and lake trout; our mountain whitefish; and our arctic grayling. Many of these salmonids are threatened or endangered and make perfect indicator species for the ecosystem as a whole; an ecosystem being pummeled by anthropogenic factors. Alberta’s Eastern Slopes streams are an endangered habitat that has already seen vast changes and will continue to degrade if we do nothing.

The watersheds in this region from south to north include the South Saskatchewan, North Saskatchewan, Athabasca, and Peace. Mountain whitefish and bull trout are native to all of the watersheds with cutthroat trout in the far south watersheds, Athabasca rainbow trout in central Alberta, and Arctic grayling starting in central Alberta and inhabiting the far north streams as well. Along with these native salmonids, there are also introduced trout and char throughout the Eastern Slopes. This region’s rivers are filled with amazing creatures but unfortunately they are on a serious decline and various factors are to blame. One of the key factors hurting the fish of Alberta’s Eastern Slopes, and the one I’ll focus on here, is habitat fragmentation.

From Wetlands Webbed Feet Not Required, Government of Alberta, 2011

From Wetlands Webbed Feet Not Required, Government of Alberta, 2011

What is habitat fragmentation?

Habitat fragmentation is when a large continuous area of habitat is divided into smaller disconnected patches. An example of this is a forest habitat that is fragmented by a highway being built down the middle. The presence of the highway will impact the movement of species from one part of the forest to another, causing the one continuous piece of habitat to be fragmented into smaller disconnected chunks. When it comes to our salmonids, the concern is blocking movement up and down rivers and streams. Salmonids rely on migration up and down waterways to move from spawning habitat to feeding habitat to overwintering habitat.

Arctic grayling are a perfect example of a fish requiring large connected stretches of habitat; they move upstream into smaller tributaries to spawn and spend the summer and then work their way downstream to larger rivers and deeper pools to overwinter when their summer habitats freeze solid. The grayling often travel more than 50 km and even in excess of 100 km to go from their wintering location to their breeding and summer location. The importance of this migration is really emphasized by the site fidelity that grayling exhibit, returning to, not just the area, but the exact location that they spent previous summers.¹ Bull trout are another species that rely heavily on connected habitat. Migratory bull trout must travel huge distances from overwintering to spawning habitat, as much as 250 km.² This opens the species up to many possible barriers as any impassable section will stop their migration in its tracks.

What fragments stream habitat?

Beaver Dams and Waterfalls

Any barrier preventing fish passage degrades the quality of the habitat for salmonids. One way that this can happen is through natural occurrences such as waterfalls or beaver dams. Waterfalls are an obvious barrier to fish movement. Although trout, char, and grayling can move up fairly large natural waterfalls — one study found 20 cm brook trout to be able to migrate up 73.5 cm waterfalls³ — if a waterfall is too large it is an impassable barrier. These natural formations restrict passage of fish but these areas have always been restricted by the formation meaning they do not cause any kind of a reduction in the populations of native fish.

Beaver dams are another natural barrier but one that is a lot more intermittent than waterfalls. Beaver dams come and go, new dams are built all the time and old dams start to leak or get destroyed in spring runoff and high water events. While a high percentage of dams do allow fish passage, actively maintained dams that have few leaks can definitely be a barrier to migrating fish. This is complicated, however, by the fact that beaver dams have been shown by many studies throughout the world to provide a net benefit for salmonid habitat through the creation of deep water refuges that can be used for rearing, overwintering, and protection.⁴ ⁵ ⁶ ⁷

Bridges and Culverts

Bridges and culverts can also be barriers to fish migration. The main infrastructure that is required to develop a region for industrial activity is roads. Alberta’s Eastern Slopes are crisscrossed with a massive amount of roads, most of which are double-lane industrial gravel roads. When roads are built, bridges and culverts are also required at the inevitable stream crossings creating barriers to fish. If these stream crossings change the stream in too great of a way, fish passage will be impeded. These changes can be:

  • An increase of water velocity

  • An increase in water turbulence

  • Too dark of an environment (think long culvert)

  • Too narrow of a passage

  • Too shallow of water

  • Too steep of an incline or even a dropoff; or

  • Too much debris or overgrowth blocking passage⁸

Hanging culvert

Any of these factors can stop fish from using the passage but the most obvious problem is steep or hanging culverts. These types of culverts do not allow any fish passage upstream and unfortunately half of the culverts in northern Alberta are of the hanging type.⁹ Other, more responsible culverts and bridges can still be a problem as they still change the environment. A 1991 Maryland study found that the erection of bridges and creation of culverts can create an impassable barrier to fish through increasing water velocity.¹⁰ An artificial change in a portion of the stream impacts the hydrology which can and does stop fish from using the passage.

When the habitat is fragmented in this way, fish are unable to move as they naturally would. Some habitat can end up completely devoid of fish and crucial spawning areas may end up inaccessible or very difficult to access, delaying the migration. Delays in migration may force fish to spawn in suboptimal habitats, decreasing the reproductive success of the species.¹¹ There is a vast amount of evidence that stream crossings fragment our Eastern Slopes and, currently, we have far too many.

Hydroelectric Dams

Hydroelectric dam

Another way that stream habitat gets fragmented is through the building of hydroelectric dams. Hydroelectric dams block the migration of fish as has been seen in bull trout in the Oldman River due to a dam constructed in 1991, the Oldman River Dam. The bull trout stack up below the dam as they attempt their fall spawning migration, fish after fish accumulating as they run into the river barrier.¹² Unfortunately there was no fish passage strategy such as a fish ladder built into this dam so the fish are unable to reach their regular spawning grounds. Some hydroelectric dams are equipped with methods to allow fish to access the other side of the dam but even dams equipped to allow fish passage block a significant number of fish. Building a hydroelectric dam in the middle of a stream impacts the ability of fish to perform their natural migration and, in turn, impacts the ecosystem and all the creatures that live there.

What can be done?

The first thing that needs to be done is stopping further fragmentation of this ecosystem. We need to stop any further stream crossings if at all possible and when there is no other way, the crossings need to be designed and built with the passage of fish in mind. This means an end to the construction of steep or hanging culverts and regulation on how crossings can be created in this endangered habitat. Further construction of dams in our Eastern Slopes also needs to be stopped. We need to press our governing bodies to ensure both of these things happen through use of the Fisheries Act and Species at Risk Act.

Flowing stream

The second thing we need to do is start to rebuild and reconnect the ecosystem by removing barriers that already exist. Stopping further degradation and fragmentation of habitat is critical but, unfortunately, not enough for such an endangered ecosystem. We also need to take steps to correct problems we have already created. This means removing problem road crossings and constructing fish passages in already present dams.

Alberta’s Eastern Slopes’ streams are an endangered habitat with many threats but a habitat that deserves to be protected. One of these threats is habitat fragmentation. Further fragmentation needs to be prevented and mistakes already made need to be fixed if we are to save this precious habitat and all of the creatures that live there.


1. Buzby, K. M., & Deegan, L. A. (2000). Inter-annual fidelity to summer feeding sites in Arctic grayling. Environmental Biology of Fishes, 59(3), 319-327.

2. Rodtka, M., Post, J. R., & Johnston, F. D. (2009). Status of the Bull Trout (Salvelinus confluentus) in Alberta: Update 2009. Alberta Sustainable Resource Development.

3. Kondratieff, M. C., & Myrick, C. A. (2006). How high can brook trout jump? A laboratory evaluation of brook trout jumping performance. Transactions of the American Fisheries Society, 135(2), 361-370.

4. White, S. M., & Rahel, F. J. (2008). Complementation of habitats for Bonneville cutthroat trout in watersheds influenced by beavers, livestock, and drought. Transactions of the American Fisheries Society, 137(3), 881-894.

5. Rahn, K. (2012). The effects of beaver dams on trout habitat in the West Branch of the Maple River.

6. Malison, R. L., & Halley, D. J. (2020). Ecology and movement of juvenile salmonids in beaver‐influenced and beaver‐free tributaries in the Trøndelag province of Norway. Ecology of Freshwater Fish.

7. Kemp, P. S., Worthington, T. A., Langford, T. E., Tree, A. R., & Gaywood, M. J. (2012). Qualitative and quantitative effects of reintroduced beavers on stream fish. Fish and Fisheries, 13(2), 158-181.

8. Cotterell, E. (1998). Fish passage in streams. Fisheries guidelines for design of stream crossings, Fish Habitat Guideline FHG, 1.

9. Park, D. J. (2006). Stream fragmentation by hanging culverts along industrial roads in Alberta's boreal forest: assessment and alternative strategies (Doctoral dissertation, University of Alberta).

10. Meehan, W. R. (1991). Influences of forest and rangeland management on salmonid fishes and their habitats: introduction and overview.

11. Stewart, D. B., Mochnacz, N. J., Reist, J. D., Carmichael, T. J., & Sawatzky, C. D. (2007). Fish life history and habitat use in the Northwest Territories: Arctic grayling (Thymallus arcticus). Canadian Manuscript Report of Fisheries and Aquatic Science, 2797, 55.

12. Fernet, D. A., & O’Neil, J. (1997). Use of radio telemetry to document seasonal movements and spawning locations for bull trout in relation to a newly created reservoir. In Friends of the Bull Trout Conference Proceedings, Trout Unlimited Canada, Calgary (pp. 427-434).

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