Cowichan Lake

CowichanLakeEastBathymetry-1990CowichanLakeWestBathymetry-1990Cowichan Lake is in the upper portion of the Cowichan Basin, at an elevation of 164 m. It has a surface area of 62 km2 (6,204 ha), is 31 km in length, with a perimeter of about 110 km. Its average depth is 50 m and its maximum depth is 152 m. For the most part, the lake bottom drops quickly from the shore except for some isolated shallows formed by the alluvial fans of tributary streams.

The Cowichan Lake Weir

CowichanLakeWeir-labelledThe weir on Cowichan Lake is used to control the outflow from the lake into the Cowichan River. For much of the time between late fall and late spring each year, it does nothing, and the weir is referred to as "off control" - since the lake level is above the top of the weir.

Rule Curve & Rule Band

The water level in Lake Cowichan is usually above the top of the weir until mid-to-late spring each year. Once the lake level declines, and until lake storage is replenished by fall rains, the lake level and outflow rate can be controlled by the weir. On April 1, generally, the lake becomes "on control", operated with two goals: to prolong lake levels through the late summer and into early fall, and to maintain a flow rate in Cowichan River of 7 cubic metres per second. 

The Cowichan Watershed

Water in the Cowichan Basin Ice-Age About 29,000 years ago, precipitation in the form of snow and a cold climate resulted in a period of glaciation that, upon its retreat about 10,000 years ago, formed the deep depression of Cowichan Lake, the much shallower depressions of Somenos Lake and Quamichan Lake, the channels of the Cowichan River and its tributaries, and the gravels, sands, and clays in the Cowichan Estuary.

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