Modeling a lake with hydraulic control at the outlet

Is there a way to model the hydraulics for a lake with a natural control at the outlet without using the spillway equation to determine the outflow. I want to specify the outflows in the branch outflow file. I tried setting KBSTR for my outflow structure at a layer corresponding with the elevation of the lake outlet elevation. I also tried placing an internal submerged weir with a top elevation equal to the outlet elevation. In both cases, the modeled water surface elevation dropped belwo the layer equivalent of the outlet elevation.