Rainy Lake Outflow Limitations - A Primer
The ability of the dam at International Falls-Fort Frances to change the rate of flow out of Rainy Lake is directly influenced by the level of Rainy Lake. At several locations upstream of the dam, features of the channel can restrict the flow from the outlet of Rainy Lake to the dam. These areas include Ranier Rapids, Pither's Point, and Koochiching Falls.
For a given lake level, there is a maximum flow rate that can be pushed through these restrictions to reach the dam (the higher the lake level, the greater the maximum flow rate). At the dam, once enough gates are open to pass water at this maximum flow rate, opening additional gates will not increase outflow - the dam simply cannot pass water faster than it arrives. Only with rising lake level will the flow rate reaching the dam increase, and only then do additional gate openings increase the flow out of the dam.
The interactive tool below, which is based on computer modelling commissioned by the IJC (see report), illustrates this concept. Follow the steps below to see the how the maximum flow rate relates to the number of gates and flow rates.
- Adjust the number of open canal gates and dam gates by clicking on the red arrows at the top. (Canal gates pass more water and are opened before dam gates).
- Adjust the level of Rainy Lake by clicking on the red arrows on the right.
- Flow past the dam is observed on the left.
- For a lower lake level, e.g. 337.5 m/1107.3 ft, adjust the number of open gates and observe the effect on flow. Opening more than two gates does not affect the flow past the dam, but does drain the dam forebay area.
- Adjust the lake to a high level, e.g. 338.0 m/ 1109.0 ft, adjust the number of open gates, and observe the effect on flow. Under these conditions, flow past the dam increases until 7 dam gates are opened.
- Adjust the lake level to a very high level, e.g. 338.6 m/ 1110.9 ft. Under these conditions, flow past the dam increases with every gate opening until the dam is completely opened.
( Interactive -- click on the arrows )
This animation was originally developed to illustrate the limitations on flow control under low-moderate lake levels, and as such cannot be used to simulate all flow conditions. Work is being done to expand this to include a wider range of lake and flow conditions.