I couldn't figure out how exactly the pond was fed, but there where probably two sources, one being the Río Ijuez and the other the run-off of the slopes behind the mill.
It's obvious that, even with such a huge capacity of the embalse, it must have been difficult to supply a reliable and regular energy flow, certainly during the often very dry summers.
It is basically a small door situated at mid height of the reservoir. It's sunk into the wall and in order to make it possible to reach it from above the stone column from the valve to the top is retracted. Why not mount the device simply on the surface instead?