From the time when I spent a lot of time around Toddbrook Reservoir, there are 4 water control methods. The main inlet stream can be diverted around the lake, there is a small low level outflow pipe, and a small weir copes with normal overflow conditions. The large auxiliary spillway is there to take extreme rainfall. The latter was replaced / installed in the 60's or 70's. Most of the time the water level is maintained well below the level of both the weir and the auxiliary spillway, and in the years I was there, I never saw the spillway in active use. I assume that on this occasion the stream inlet control was overwhelmed by the sheer amount of water, hence the need to dump bags of aggregate there.