Grass River

The Grass River is a river on the edge, geographically and metaphorically. Granite meets limestone. Uplands meet lowlands. Black spruce and jack pine meet white spruce and birch, and murky, tea-stained waterways meet transparent, turquoise-blue lakes.

Like an outline on a physiographic map, the Grass River defines the meeting point of two geological zones and ecoregions. It marks the uneven boundary between the ancient North American Precambrian Shield – commonly known as the Canadian Shield – and the vast boreal plain of the Manitoba Lowlands, underlain by flat beds of softer sedimentary rock.

To the north and west, where the river washes over the Shield, it barely covers the granite bedrock. Its shallow lakes and pools are sprinkled with protruding islands and rocky outcroppings, and the water has a brownish tinge, darkened by the tannic acid that seeps from its geological foundation. But along its southern shore, where the Grass River skirts the northern edge of the Manitoba Plain, its lakes sparkle with a brilliant blue. Their limestone bottoms act to neutralize the acids and keep the water clear.

The geological dividing line of the Grass River also corresponds to differences in ecological character. While most of the waterway lies in the ecoregion known as the Churchill River Uplands, its southwestern reaches dip south into the Mid-Boreal Lowlands.

Vaguely hinting at a harsher, colder climate to the north, a wetland complex in Grass River Provincial Park, in the southwestern watershed of the Grass River, contains fens, peat palsas and palsa scars. Palsas are peat mounds, 1.5 – 6 meters high and up to 100 meters in diameter, with permanently frozen cores. The Grass River palsa site marks the southern limit of the discontinuous permafrost zone, more commonly found in the Hudson – James Bay Lowlands area. The Palsa Hazel Ecological Reserve is one of 16 provincially–designated reserves in Manitoba. Ecological reserves preserve unique and representative plants, animals and geological features, natural landscapes and ecological processes, and serve as outdoor laboratories. Recreational activities in these highly protected areas are prohibited.

One of the greatest contrasts between the granite bedrock of the Grass River’s northern reaches, and the limestone pavement of its lower watershed, is permeability. While the hard rock of the Canadian Shield resists the penetrating effects of rainfall, the exposed sections of the carbonate bedrock of the boreal plain can dissolve, crumble, and collapse. A series of underground caves and channels in the lower reaches of the Grass River marks the areas where the limestone of the lowlands has begun to give way to the forces of nature.

The Grass River country also possesses prime habitat for caribou: old-growth forests with abundant lichens, islands, and muskegs for protected, predator-free calving, and a sparsely-distributed human population. The Grass River watershed is within the range of Manitoba’s boreal forest woodland caribou, a 14-herd population that totals 2,000–2,500 animals. Although the Manitoba caribou herds do not appear to be experiencing the steep declines of several western caribou populations, there is increasing concern that mining, forestry, and other industrial developments may place some herds at risk.