The Agri-Drains come in multiple sizes in 1 foot increments up to 12 feet in height. I purchased an 8 foot box. The water level is controlled by installing or removing slats inside the box. The slats are alternating 5" and 7" in height which allows precise water level control which is very important in some duck habitat installations. Mine was installed where with all slats installed I can raise the water level to the spillway height. I plan to maintain a normal freeboard of 1 foot below spillway to begin with until I see how that works out. I will also be able to draw down the water level a full 8 feet below splillway for winter time or dock maintenance etc. I thought the additional flexibility was worth the extra expense for the box. I suppose time will tell on my decision.
