Cowichan 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.
About two dozen streams supply Cowichan Lake; the largest being Meade, Cottonwood, McKay, Shaw, Nixon, Sutton, Ashburnum Creeks, and Robertson River.
Thirteen of the fish species that occur in the watershed are found in the lake. These include anadromous bull trout, atlantic salmon, brook trout, brown catfish, brown trout, Chinook, chum and coho salmon, cutthroat and rainbow trout, dolly varden, kokanee, and western brook lamprey.
About 6000 humans live in the lake area and the lake is a swimming, boating and fishing recreational paradise for thousands if visitors each year.
The lake serves as the source of drinking water for the Town of Lake Cowichan.
It is also the reservoir for the Cowichan River, downstream water users and the provision of ecological services, particularly for migrating salmon. A weir was built at lake Cowichan in 1957 to better manage the reservoir function, specially for late summer - early fall conditions when river discharges can fall below the growing demand downstream.
Low summer flows and increasing demand in the intervening 50+ years have put the issue of matching water availability with demand in the forefront again.
Westland Resource Group, October 2005
The Importance of Cowichan Lake and Its Watershed
BC Lake Stewardship Society, December 2008