I haven't re-read the article, but I recall the control gates were at the bottom of the dam. Traditionally the spillway is at the top of the dam and on one port

I haven't re-read the article, but I recall the control gates were at the bottom of the dam. Traditionally the spillway is at the top of the dam and on one portiona there are gates which are operated hydraulically or by winches that open to allow water to pass underneath and run down a specially shaped (ogee shape) face of the dam. If for some reason the water level behind the dam gets too high there can be a section of spillway with no gates that will allow water to flow around the dam without the dam being overtopped in an uncontrolled fashion. The mosul dam is 113 m high and the article said they were operating the dam with the water level 30m below the normal full level. Probably the sill of the spillway is only 10 or less below the normal operating level, because if the dam isn't full why would you want to spill. The control gates at the bottom of the dam could have been primarily for allowing sediment to be sluice out from behind the dam or to allow the reservoir to be drained. There is no guarantee the two control gates could pass as much flow as the turbines. Who knows how they were sized. As I tried to explain before, cutting the power lines shouldn't damage any equipment. It effectively happens all the time due to natural causes, be they tree branches falling on power lines, birds, squirrels, etc. Flow through an orifice (sluice gate, control gate, turbine, etc) varies with the square root of the pressure of water (the head), so a side effect of operating the dam with 30 m less head (30% less!) is that the turbines would be operating outside of their normal range and the control gates would pass less flow than they would at the nominal head. There was a terrible accident at a dam in russia that was caused by operating the turbine outside of the normal range of pressure, and poor maintenance (loose bolts holding the turbine down...): https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenskaya_power_station_accident https://en.wikipedia.org/wiki/2009_Sayano%E2%80%93Shushenska...