I'm now inclined to think that it doesn't have a dam in the normal sense - the topography doesn't suggest that it does, and in fact it may have originally been a natural lake in a depression.

I added a small lock on the northwest side of the lake connecting it to the ditch there, and that seems like more clearly an inlet. The larger canal marked "Windsor Lake Inlet" appears to be bidirectional - Windsor Reservoir Outlet clearly flows SE, but the inlet spur doesn't have any signs, even debris buildup, suggesting that it goes one way or the other, but rather that it just connects the lake to the ditch, and they maintain a similar elevation.

I made a bunch of other small tweaks to alignments and the north lock, but at this point I'd be OK simply removing the dam since it doesn't appear to have one. Maybe a very old map could add more clarity, however.