One unique feature of the lake is the Jasper Lake Sand Dunes. It is the only sand dune ecosystem in the Canadian Rockies. The dunes were formed at the edge of the lake during the last ice age and have been constantly reshaped by wind and water ever since. Acting as a kind of sieve, the lake removes silt and sand from the river, allowing it to sink to the lake bed. When the water level recedes in the fall, vast sand flats are exposed and dry out, becoming vulnerable to the strong westerly winds that sweep through the lower Athabasca Valley. In winter, these winds blow the sand and silt down the valley, forming two large dune islands near the northwest shore of Jasper Lake. In the lee of the dunes, mature clumps of spruce and balsam poplar have gained a stronghold, while colonizing grasses, rose bushes and willows wage a constant war against the winds and migrating sands. Nowhere else in the park is the balance of nature so apparent - or so fragile - as in the Jasper Lake sand dunes. As water from the river flows into the lake, silt and sand sink down to the lake bed. In Autumn, when the water level goes down, the silt and sand are exposed and become dry, and are then swept up by the strong westerly winds of the Athabasca Valley.