Yes, that's the spillway.

- if the purpose of the dam is to supply drinking water then the intake could be lower to allow using all the available supply. That though would mean that the demand exceeds the supply. Once the community lowers the water level to the bottom of the intake they would then have to depend on the rate of flow of the water supply to refill the dam .. not very appealing.

- if the purpose of the dam is to raise the water high enough to power a turbine then the higher the better. If you take the flow volume of the stream .. cross-section of stream - 1 foot x height x width .. x .. rate of flow (like 1 ft per second) then .. x .. 60 you get cu.ft. per minute. Water weighs 62.5 lbs per cu. ft. That's your power source .. that weight raised. The higher you raise it the more work you get from it. Just calculating that .. lbs per minute without any head (raising the water up) you divide by 33,000 and get hp from that water.

The interesting thing about this is that you can take a stream that might not have enough energy to power a turbine at ground level can be successful simply by raising it up via damming. That's from a book titled '

Power development of small streams' published in 1920. Example .. a 12 inch Hunt-Francis turbine (No.2 wheel) would produce only 1.15 hp with a 4 ft head requiring 187 cu.ft. min of water. If you raised the head to 87 ft that jumps to 119.24 hp requiring 873 cu.ft. min water flow. 873 cu.ft. min isn't really that much .. that would be for example a stream 7 ft wide and 2 ft deep flowing at 1 ft. per second.

Ok. Sorry for all the numbers .. but I've been happily calculating the requirements for my little dam to make sure all the 'bits and pieces' are the right sizes