Multi-Purpose dams hold water for various seasonal, domestic and irrigation uses. Each is designed to regulate seasonal storage, to control spring and summer flood water and to store water for summer use. Stockwatering, wildlife habitat, and fish rearing are other common uses. The dams are totally drained in the fall to prepare for full flood control potential in the spring.  

The figure below shows the control valve and the vertical drop pipe. Within the dam the vertical drop inlet pipe connects to the main horizontal drain to serve as an operating spillway. The result is that the water level should only rise to the top of the drop inlet pipe which is 50% of the storage capacity of the dam. When the water level rises above the verical pipe, then the water runs down the pipe.

The image below shows the cross-section at high water. The dam will fill to the desired level (50% of storage capacity) and then excess water drains into the culvert from above. The drop inlet is the "operating spillway", and it is in place to prevent water from overtopping the dam.

Here a picture of a multi-purpose dam. Notice the control gate standing out of the water and the grill on the top of the vertical culvert. The grill is to keep debris and beavers out of the pipe.

Here is another view of the multi-purpose dam. The contol and trash rack can easily be seen. Vegetation has not yet been established around this dam.

This animation shows the water rising behind a multipurpose dam when the control valve is closed. The water level will stop at the top of the vertical control pipe.

http://www.cici.mb.ca/deerwood/dams04.html