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Most of us are aware that water levels on our lake are controlled to some extent, but we wonder why flooding occasionally occurs, and sometimes levels drop late in the summer.  There is a dam at Carleton Place, so why can't the lake be maintained at a constant level?  There is no simple answer because the problem is quite complex.

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This lake is part of the Mississippi River watershed, a vast network of lakes, rivers and streams with a drainage area of 3750 sq. km. The river is 212 km long and there are approximately 268 lakes. Most of the water that feeds this system arrives during a short period of time during the spring runoff, which can lead to much increased water levels and flooding.  However, for most of the year, the average rainfall in the area is not sufficient to provide flows to maintain suitable lake levels, especially on Dalhousie and Mississippi Lakes.  Various dams on the watershed are operated by MVCA to manage these two conflicting situations, (and many other issues including ecological, wildlife habitat, erosion, etc.). 

 

Mississippi River Water Management Plan (MRWMP)

The Mississippi Valley Conservation Authority has the task of managing the water levels in this watershed.  Although an informal operating plan had been used over the years, a formal Mississippi River Water Management Plan was developed and approved for use in June, 2006.

 

The Final Report of this plan is an extensive study of the numerous factors affecting water management in this system, and is an essential reference source for understanding the issue. Ed Carew, a former MLA Director and Vice President, was one of the 12 Public Advisory Committee members for the Mississippi River Water Management Plan for the Water Power project, and has served as Chair of the Standing Advisory Committee (SAC).

 

While the MRWMP was undertaken because hydro facilities exist on the Mississippi, hydro production is not the driver for water management operations.  During the preparation of the Mississippi River Water Management Plan much consideration was given to an integrated approach to maximize all uses of the river including water power, flood control, low flow augmentation, fish and wildlife, tourism and recreation.

 

Carleton Place Dam Operation

This is of particular interest to residents of Lake Mississippi and the communities immediately downstream.  The operating range for the Carleton Place Dam is between 133.93 m and 134.50 m. Within this range, the dam is operated to achieve a summer target level on Mississippi Lake of 134.35 m by the long weekend in May and to maintain a target range of 10 cm above and below the optimum level for Mississippi Lake, from the end of May to the start of the following freshet (spring runoff).

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Due to constrictions in the natural channel at Bridge Street in Carleton Place, which has less conveyance capacity than the Carleton Place Dam, water levels on Mississippi Lake cease to be influenced by the operation of the dam once 25 stop logs have been removed and/or stream flows have exceeded 150 cm. This will result in water levels on Mississippi Lake normally ranging from 134.9 m in the spring to 133.95 m in the fall.  Stream flows in excess of 150 cms will result in higher water levels on Mississippi Lake.

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MVCA provides a detailed explanation of the general operating principles for dams on the Mississippi River.

 

Floodplain Considerations

Mississippi Lake is a relatively shallow lake with a substantial portion of low lying shoreline. During periods of heavy precipitation or during the Spring freshet, the shoreline is vulnerable to flooding. Important components of watershed management are the identification of these vulnerabilities and the development of regulations to minimize risk and damage to life and property. Accordingly, the MVCA has undertaken an extensive floodplain mapping project, and provides excellent information on their website regarding flood prone areas, flood warning messages, and regulations for building on a floodplain.