The navigation pools in the St. Louis District are maintained based upon hinge point limits. As the flows increase and the upper limits of the hinge point is approached, the pool level is lowered to compensate until the flows start decreasing or max drawdown is reached. As the flows decrease and the lower limits of the hinge point is approached, the pool level is raised to compensate until the flows start increasing.
Limits of the hinge points are defined based upon:
Authorized U.S. government easements
The lower limit at the hinge points are defined by the lowest river level allowable for a safe and dependable navigation channel and in coordination with dredging efforts. The upper limit is defined by the highest river level that can be maintained by the dam because the land above this limit isn’t owned by the U.S. government, thus not authorized to be inundated by the project.