St. Mary Reservoir is located 45 kilometres southwest of the city of Lethbridge, with the primary access to it being via Secondary Highway 505 which crosses the dam at the northeastern corner of the reservoir. The reservoir itself is a large water storage area (19 km by 8 km at its widest) within the St. Mary River valley; the water is clear and relatively algae-free. When the reservoir level drops by its mean annual drawdown of 6.7 metres, 46% of the reservoir area is exposed as mudflats. There are also five permanent islands near the upstream end, and other islands appear as the water level drops. Aside from the reservoir itself, there is some natural short-grass prairie and agricultural lands in the surrounding area. The soils are black chernozemic while the underlying bedrock is mainly sandstone, mudstone, siltstone and thin coal and limestone beds. The terrain around the reservoir is mostly gently undulating to flat.
The most important species found at the St. Mary Reservoir is the California Gull there is a colony here of over 2000 nests, which is globally significant (just over 1% of the global population). That count, however, was before 1990, while the most recent survey in 1998 revealed only 307 nests (not globally significant). But the 1998 numbers are low compared to other years as a result of low nesting success caused by severe storms and heavy prolonged rains. More surveys will hopefully reveal a typical average for the colony. For instance, in 1975, almost 5,000 combined nests of California and Ring-billed gulls were recorded. Another species, the American White Pelican, is sometimes present during summer in nationally significant numbers. Over 1,000 of these birds were counted on June 23, 1995, on the reservoir (about 1% of the Canadian population).
The St. Mary Reservoir has several other colonial nesters. The 1998 colony survey revealed 15 Ring-billed Gull nests , 76 Double-crested Cormorant nests, and one American White Pelican nest. Again, these numbers are thought to be low compared to other years due to the weather. Piping Plovers were found in 1996, with one pair and one unpaired adults recorded. Non-breeding Eared Grebes and Common Terns are seen in summer, with over 100 and over 50 respectively seen each year.
BirdLife International (2021) Important Bird Areas factsheet: St. Mary Reservoir. Downloaded from http://www.birdlife.org on 28/11/2021.