I am interested in a couple of different water control structures.

The easiest and cheapest thing to do is just have a cap or valve. If I want to just plant a crop and then flood it that would be the way to go.

Another type are flashboard risers. With these you can set the depth you want and not have to worry about maintaining the correct level. By removing a board you can drop the water level slowly which is best.

Here are the flashboard risers that I am looking at.

The whole setup with pipe, riser, and seep collar will be about $700. The riser will have a 4" pipe and either 4-5' tall.
http://agridrain.com/watercontrolproductsinline.asp

This setup will be around $1200. The riser will be 4' tall with a 1' pipe.
http://5oakswildlifeservices.com/?page_id=6