Oak Hammock Marsh

Oak Hammock Marsh consists of approximately 20 square kilometres (7.7 sq mi) of open marsh, and a slightly smaller area of surrounding woods and grasslands. The total Wildlife Management Area is 36 sq km. It is located approximately 20 km directly north of Winnipeg, Manitoba, Canada, near the town of Stonewall, Manitoba in the Rural Municipality of Rockwood.

Oak Hammock Marsh is listed as a Wetland of International Importance under the Ramsar Convention, and is also a globally significant Important Bird Area.

The marsh is a remnant of an originally 470 sq km area of marsh and fen near the south-western corner of Lake Winnipeg. This larger marsh had the original English name of St. Andrews Bog. But the original wetland underwent drainage for agricultural purposes beginning in 1897, and by the early 1960s all but 60 ha had been drained. Measures to restore a portion of the wetland began in 1967 when the Governments of Manitoba and Canada embarked on a cooperative program with Ducks Unlimited Canada and other wildlife conservation organizations to restore marginal agricultural lands to a state suitable for wildlife.

By 1974, 3,450 ha of land had been purchased and 22 km of dykes built to trap and hold water in three dyke-separated marsh compartments. In addition, 58 nesting islands were constructed within the three compartments. In 1984, the Manitoba government signed a further development agreement with Ducks Unlimited Canada to construct water control structures, water supply works, more nesting islands, additional dykes, and create a fourth compartment.

The marsh is a re-constructed and managed wetland, designed for the creation of a waterfowl breeding and migratory habitat. Water levels in the marsh are carefully controlled. It is common during wet years (when waterfowl have an abundance of alternative nesting sites) for the water level in one or more of the compartments to be lowered for the summer, creating an extensive area of dried mudflats. This drying and later reflooding promotes the growth of emergent marsh plants such as bulrush and cattail, and therefore maintains the vegetation cover of the marsh; otherwise, the natural tendency would be for the marsh to become over several years simply a shallow lake, with a sharply defined shoreline and little nesting cover. Furthermore, not all of the adjacent purchased land has been allowed to grow wild. Cereal grain crops are planted in some of it, in order to supply migrating waterfowl with an autumn food supply while reducing crop losses on local farms.