The summed pressure of the Mississippi water on the vast palisade must have been huge, but the pressure on any one single timber—they were about the size of railroad ties—was modest enough that it could be pulled up to create a slit that would let some water through. Rail-mounted cranes moved along the top of the frame pulling the timbers up or pushing them down to regulate the volume of flow diverted from the river into the sluice and thence to Lake Pontchartrain, which had its own outlet to the Gulf of Mexico.