What I was missing is that in the Panama canal the high point is in the middle, not at one end, so the water flow is from middle to both ends. In a normal lock

What I was missing is that in the Panama canal the high point is in the middle, not at one end, so the water flow is from middle to both ends. In a normal lock system (e.g. typical British canal) the water flow is from the high end of the entire set of locks to the low end, so gravity does the job, and the water supply at the high end is replenished by whatever it is connected to (river or lake).