Keegs, consider using 4" PVC pipe and a valve about 6' deep on the backside of the dam for year round level control or draining if needed. The pool side of the pipe can be covered with large gravel on the outflow side should be in a location that won't erode, plus covered with a critter cage or flap cover to prevent animals getting stuck in it. A siphon would be better for all around flow control and main drainage, but I am not sure it wouldn't freeze and become useless in your area.
