St. Mary Reservoir

The contents of this online version has not been altered or modified from the original 1990 publication. It is reasonable to assume that much of the data e.g. water levels, camp grounds/boat launches, etc. is out of date. For updated or additional information on any of the lakes in this atlas please go to Environment Alberta's water web site.

Basic Info
Map Sheets82H
Lat / Long49.3000000, -113.1833333
49°17'N, 113°10'W
Area37.5 km2
Max depth56.4 m
Mean depth10.4 m
Dr. Basin Area2250 km2
Dam, WeirDam
Drainage BasinOldman River Basin
Camp GroundPresent
Boat LaunchPresent
Sport FishLake Whitefish, Walleye, Northern Pike, Rainbow Trout, Cutthroat Trout
Trophic StatusOligotrophic?
TP xNo Data µg/L
CHLORO x2.9 µg/L
TDS x114 mg/L
Photo credit: unknown


St. Mary Reservoir is a large water storage reservoir that fills the valley of the St. Mary River 45 km southwest of the city of Lethbridge in the Municipal District of Cardston. To reach the reservoir from Lethbridge, follow Highway 5 for 53 km southwest to the village of Spring Coulee. Turn west on Secondary Road 505 and drive for 7 km to the dam (FIGURE 1), which can be crossed by vehicles. Access to the reservoir is available at two boat launches near the dam, one on the north shore and one on the south shore (FIGURE 2). It is not possible to launch a boat at either site when the water level is more than 7 m below the full supply level. All boats are restricted from posted areas in the reservoir (Alta. For. Ld. Wild. 1988). There is a day-use site on the southeast shore just east of an island that shelters the beach from waves; facilities include portable change houses. St. Mary Dam Park (also known as Spring Coulee Park) is located along the river immediately downstream of the dam. This park, operated by Alberta Environment, provides a 32-site campground, a picnic shelter, tables, drinking water and a basketball court (Alta. Envir. n.d.[a]). To provide angling, Fish and Wildlife Division stocks a small pond in the park each year with rainbow and brook trout.

The reservoir was named after the river, which in turn was named after the two St. Mary lakes at its headwaters in Glacier National Park, Montana. These lakes were named by Father Pierre Jean de Smet (1801 to 1873), an early missionary in the west (Holmgren and Holmgren 1976). In about 1750, Blood Indians moved from the Hand Hills near the town of Hanna to the present location of the reservoir. In 1869, a fur trading post, Fort Whoop-up, was built near the confluence of the St. Mary and Oldman rivers. By the 1880s, cattle ranching was a major occupation; the area became the site of large ranches, such as the Cochrane Ranch west of the Belly River and the McIntyre Ranch on Milk River Ridge (Magrath Dist. Hist. Soc. 1974; Cardston Dist. Hist. Soc. 1978). In 1884, the Blood Indians moved onto the Blood Indian Reserve west of the St. Mary River (Cardston Dist. Hist. Soc. 1978).

This area of Alberta receives little rainfall and is almost continuously blown by warm, drying winds that drop from the mountains. The need for irrigation was soon recognized, and members of the Mormon community in Utah, with many years of experience with irrigation, were encouraged to settle in the area. As an incentive, they received cash and land from the Alberta Railway and Irrigation Company (Magrath Dist. Hist. Soc. 1974). By 1890, many small irrigation projects were in place, and in 1900, the "Great Irrigation Canal" was built to divert water from the St. Mary River southeast of the town of Cardston to the Lethbridge area (Cardston Dist. Hist. Soc. 1978).

There were no major storage reservoirs west of Lethbridge until after World War II. In 1946, the Prairie Farm Rehabilitation Administration (PFRA) of the federal government began construction of the St. Mary Dam, then the largest earthfill dam in North America. The reservoir was first filled in 1951 (Magrath Dist. Hist. Soc. 1974). In 1959, a canal was completed to bring additional water from the Belly River to St. Mary Reservoir, and in 1960, another canal was completed to divert water from the Waterton River to the Belly River for diversion to St. Mary Reservoir (Clements 1973). The PFRA operated the reservoir until 1973 when it was turned over to Alberta Environment. More than half of the water leaving the reservoir flows through a system of canals that supplies water for irrigation, domestic, municipal and industrial uses, and to support wildlife, fisheries and recreational uses as far east as Medicine Hat. In 1988, St. Mary Reservoir provided water to irrigate approximately 190,000 ha in areas that reach as far east as Medicine Hat (Alta. Envir. n.d.[a]). The map in the introduction to the South Saskatchewan and Milk River basins shows the canals and reservoirs along the distribution system for St. Mary Reservoir.

St. Mary Reservoir is used for recreational power boating, water skiing, wind surfing, swimming and sport fishing for northern pike and walleye. Provincial sport fishing regulations apply to the reservoir and angling with bait is not permitted in the trout pond in St. Mary Dam Park (Alta. For. Ld. Wild. 1989). The reservoir also supports a commercial fishery for lake whitefish. The water is clear, relatively algae-free and attractive. However, strong winds and large waves inhibit some recreational uses, and when the reservoir is below full supply level, extensive mud flats detract from its appeal.

Drainage Basin Characteristics

St. Mary Reservoir is an onstream reservoir, and therefore its basin is that of the St. Mary River, which extends south into Montana, U.S.A. and west to the continental divide (FIGURE 1). The basin of the St. Mary River includes portions of five ecoregions (Strong and Leggat 1981). Most of the Canadian portion of the basin is in the Fescue Grass Ecoregion. Natural vegetation is dominated by rough fescue grass with secondary quantities of Parry oat grass; shrubs such as buckbrush and saskatoon grow on north-facing slopes of coulees and in seepage sites. Soils are typically Black Chernozemics, which are excellent for crop production, and much of this ecoregion is cultivated for cereal crops. A small portion of the Aspen Parkland Ecoregion is located along the lower slopes of the foothills along the border between the Aspen Parkland and Fescue Grassland ecoregions. Trembling aspen forms more than 15% of the cover. The balance of vegetation is rough fescue and shrubs such as buckbrush, saskatoon, rose and willow. Soils are Black and Dark Brown Chernozemics. West of the Aspen Parkland is the Montane Ecoregion, where Douglas fir is interspersed with grasslands on south-facing slopes. Soils are Black Chernozemics and Eutric Brunisols. As the basin reaches up into the mountains, the Subalpine Ecoregion is encountered. Vegetation is typically lodgepole pine, white spruce and fir underlain by Eutric Brunisolic soils. South of the Canada-United States border, the drainage basin reaches up to the Alpine Ecoregion, which lies above the treeline. Vegetation is absent from rock peaks that reach an altitude of 3,100 m; lichens, heaths and dryads grow in lower areas, and in sheltered places, stunted alpine fir, Engelmann spruce and alpine larch may grow. Soils are Regosols and Brunisols.

One of the most noticeable aspects of the St. Mary Reservoir area is the wind. It blows down from the mountains to the west almost continuously, and only 2% of the days in a year are calm. Wind speed averages 33 km/hour, but gusts over 170 km/hour have been recorded. The flat topography and scarcity of tree cover allow the wind to sweep the area unimpeded. Dunes have formed in places along the south shore of the reservoir, and when the reservoir is drawn down, fine particles of sand lift off the exposed mud flats to create dust storms (MTB Consult. Ltd. 1977).

The major land use in the Canadian portion of the drainage basin is agricultural-both crop production (cereals and alfalfa) and ranching. There are two small provincial parks in the basin, Woolford and Police Outpost. About one-half of the American portion of the basin lies within Glacier National Park; the major land use outside of the park is ranching. The population of the basin is low. There is one town, Cardston, in the Canadian portion and two towns, Babb and Saint Mary, in the American portion. The major water use of the St. Mary River in both countries is irrigation. An annual average of 216 x 106 m3 is diverted out of the river on the American side of the border (TABLE 2).

The only Crown land near the reservoir is the reservoir right-of-way (FIGURE 2). Most of this land is leased to local ranchers for grazing or cultivation. The entire north shore, outside the right-of-way, is part of Blood Indian Reserve 148.

Lake Basin Characteristics

St. Mary Reservoir has a large surface area relative to the size of its basin (Tables 1, 2). The lake is 19.2-km long and 8.0-km wide at its widest point. The lake bed is that of the St. Mary River, which is deeply incised into the surrounding plain. The lake bottom drops steeply to a maximum depth of 56.4 m near the dam. In the main body of the reservoir the bottom slopes more gently to a maximum depth of 30 m (FIGURE 2). When the reservoir level drops by its mean annual drawdown of 6.7 m (TABLE 2), 46% of the reservoir area is exposed as mudflats (MTB Consult. Ltd. 1977). There are five permanent islands in the reservoir, all near the upstream end. Other islands appear as the water level drops below the full supply level.

The topography of the shore of the reservoir includes areas of flat floodplain near the western end, gradual slopes along much of the southern shore, cliffs up to 7 m high (formed by wave erosion) in areas exposed to the west wind, and areas of slumping along portions of the southwest shore (MTB Consult. Ltd. 1977).

Fifty-nine per cent of the inflow to the reservoir comes from the St. Mary River and 41% comes from the Waterton and Belly rivers via the Belly-St. Mary Canal (TABLE 2). Tunnels under the dam direct 68% of the outflow to the St. Mary-Milk River Ridge Main Canal; the remaining 32% of the outflow forms the downstream portion of the St. Mary River (Alta. Envir. n.d.[a]).

The water level of the reservoir undergoes wide fluctuations. The mean annual drawdown from 1952, the first year of operation, until 1985, was 6.7 m below the full supply level. The lowest level during this period was 12.1 m below the full supply level (TABLE 2, FIGURE 3). In 1987, after 3 very dry years, the water level was drawn down a record 24.4 m below the full supply level (Alta. Envir. n.d.[a]). The reservoir is operated to fill quickly from early April to June; the level is held steady from June to early July, then falls from early July to late October (FIGURE 4).

Water Quality

The water quality of St. Mary Reservoir was monitored in 1983 and 1985 by Alberta Environment (Alta. Envir. n.d.[b]; Charlton and Brennan 1986).

St. Mary Reservoir has fresh water (TABLE 3). The alkalinity is low compared to that of natural prairie lakes. The dominant ions are calcium and bicarbonate. In 1985, the deepest area of the reservoir was weakly thermally stratified from May through August (FIGURE 5). The wind and strong current from the inflow likely inhibit stratification in the shallower upstream area. Oxygen depletion occurred with depth but anoxic conditions were not found (FIGURE 5).

The reservoir has low chlorophyll a concentrations, indicating very low algal biomass (TABLE 4, FIGURE 6). St. Mary Reservoir is likely oligotrophic, but more chlorophyll a and phosphorus monitoring is needed to confirm this.

Biological Characteristics


There are no recent data on the phytoplankton in the reservoir. The extreme annual drawdown and high wave action have kept the reservoir shoreline clear of macrophytes.


There are no recent data on the invertebrates in St. Mary Reservoir.


Thirteen species of fish have been found in St. Mary Reservoir (Clements 1973; English 1977; R.L. & L. Envir. Serv. Ltd. 1985). Northern pike are indigenous and provide the bulk of the sport fishery. Walleye were introduced in 1957 and now persist as a small, self-sustaining population that contributes to both the sport and commercial fisheries. Rainbow trout, which are indigenous to the basin and were stocked in the reservoir in the 1950s, are caught occasionally. Burbot and cutthroat trout, which are also native to the system, are caught occasionally as well. Lake whitefish are not native to the system, but were introduced to St. Mary lakes in Montana before 1910; they subsequently migrated downstream to the reservoir and are now the major target of the commercial fishery. Other fish species in the reservoir include white and longnose suckers, and five forage species: trout-perch, spottail shiners, lake chub, fathead minnows and longnose dace.

The commercial fishery has operated since 1962/63. Between the 1968/69 and the 1987/88 seasons, an average of 8,336 kg of fish were caught annually: 88% of the catch was lake whitefish, 10% was pike and 1% was walleye. Lake whitefish are free of the parasite Triaenophorus crassus (Alta. For. Ld. Wild. n.d.).

A pond is located in an isolated oxbow immediately below the dam. The continuous flow of water seeping from the dam provides clean, cold water and the pond is stocked annually with yearling rainbow trout that provide an intensive sport fishery. From 1980 to 1987, an average of 2,500 rainbow trout approximately 14-cm long were stocked every spring (Alta. En. Nat. Resour. 1980-1985; Alta. For. Ld. Wild. 1986; 1987). The use of bait fish in this pond is not permitted (Alta. For. Ld. Wild. 1989).


The water level fluctuations of St. Mary Reservoir are too great to provide nesting habitat for most waterfowl, but in 1977, Doublecrested Cormorants nested there (Markham 1978) and Canada Geese were seen nesting on one of the islands (MTB Consult. Ltd. 1977).

The area of grassland not disturbed by agriculture is small, and mammals are limited to mule deer, ground squirrels, skunks, long-tailed weasels, mice, voles and an occasional coyote. Upland game birds such as Sharp-tailed Grouse and Hungarian Partridge have been observed in the area (MTB Consult. Ltd. 1977).

J.M. Crosby


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