If you don't use pumps you need your whole supply of water to be sourced from the hight of the high point in the locks. How would you propose to operate a pure

If you don't use pumps you need your whole supply of water to be sourced from the hight of the high point in the locks. How would you propose to operate a pure gravity lock system without an outside supply of water? With the Panama canal, the crucial difference is that a large part of the canal is Lake Gatún, a freshwater lake, so they can't pump water up to avoid overdrawing the water supply