General Operating Principles for Dams along the Mississippi River
All of the dams in the western portion of the watershed were originally built to maintain enough water in the system to allow timbers to be floated downstream three or four times a year. The purposes of these structures have become diversified with the changing conditions in the watershed. As well as flood protection, low flow augmentation, ice management, erosion control and recreation necessary as a result of extensive development along the watercourse over the last fifty years, they also must be operated to maintain specific flow and level requirements for lake trout, walleye, bass, pike and as much as possible, all other fish species. Stable levels are required for wildlife such as loons, frogs, muskrat and beaver and sufficient flow maintained to allow hydro producers to continue operating their plants and turn a profit.
The management of the system has become increasingly difficult as weather patterns alter when and how much water is available at any given time, storms and droughts have tended to be more severe and new restrictions are placed on how dams can be operated. This watershed historically receives approximately 870 mm of precipitation annually and it loses approximately 530 mm of that to evaporation annually, thereby leaving only 340 mm to re-supply groundwater aquifers, fill the upper lakes and maintain a minimal flow of 5 cubic meters per second (cms) throughout the year.
There are six major lakes in the watershed which act as storage reservoirs in the spring to alleviate flooding downstream of Crotch Lake. These lakes – Shabomeka, Mazinaw, Kashwakamak, Big Gull, Crotch and Mississagagon – all have water control structures at their outlets. There are two other notable lakes on the main branch of the Mississippi River, being Dalhousie and Mississippi Lakes. Neither of these lakes have a dam at their outlet although Mississippi Lake is influenced by the Carleton Place Dam under low flow conditions. Both of these lakes are heavily developed.
Every fall, the dams are operated to drawdown the lakes to provide storage for the spring runoff. As snowmelt and spring rains occur, the lakes are gradually filled to reach the summer target levels for recreation and tourism. It requires approximately 140 mm of runoff from rainfall and snowmelt to fill the lakes. Conditions must be monitored to ensure that the targets can be reached while ensuring adequate storage remains for late spring rainfalls and sufficient flows and levels are maintained for spawning fish. In doing so, there is a reduction in flooding to areas downstream as the uncontrolled flows from Antoine and Cranberry Creeks, the Fall and Clyde River systems move through the central and eastern portion of the watershed. Once the runoff is over, all of these dams, except for the Crotch Lake Dam, are operated to maintain relatively stable elevations on the lakes for recreation throughout the summer months. Crotch Lake Dam is unique as it is the only true reservoir lake on the system.
From late June through early October, Crotch Lake is drawn down to ensure flows in the lower portion of the river. Under normal conditions, approximately 60% of the flow in the river comes from Crotch Lake. During the droughts of 2001 and 2002, 100% of the flow in the river below Crotch Lake came from Crotch Lake as all other tributaries had virtually dried up. The upper lakes were operated to bring levels down to the bottom of their respective target range to maintain flow in the river in the western portion of the watershed. Crotch Lake normally fluctuates from 2.5 to 3.5 m (depending on amount of precipitation) over the course of the summer.
Throughout the fall, as the other lakes are being drawn down, Crotch Lake is filled again while still maintaining at least a minimum average flow of 5 cms downstream of the dam. From January through March the lake is again drawn down to perform the same low flow augmentation function over the remainder of the winter months and to maximize storage in the lake for spring.