Would the evaporation rate at the end equal the volume of water doing the 400 meter drop?
I saw something like that in at the Alcan dam up north of Chicoutime, Quebec. A lake in the mountains with 2 60' tunnels from the dam running under the mountain and having a drop of over 1500 meters to the generators. The Power station was 500 feet underground behind pressure doors and the run after the generators was tunneled for another 3 miles with another 2000 meter drop to the Sagnuay River.
Nothing visible on site but an elevator shack and huge lines comming out of the ground and running a hundred miles to power the huge bauxite smelting operation (largest in the world) at Chicoutime. Also, the left over electicity supplies all the needs for the city of Chicoutime.