Does flow through the spillway of a full reservoir increase flood levels downstream?
No, flow through the spillway of a full reservoir does not increase flood levels downstream. When a major runoff event occurs, rivers without reservoirs convey runoff in a free flowing, unobstructed manner. On the other hand, if a river enters a reservoir, river flow then spreads out over the wider width and greater depth of the reservoir. This acts to slow the flow of water over the length of the reservoir. Even though the total amount of water passing through the reservoir is the same, this delaying mechanism reduces flood heights below a reservoir in almost all circumstances.
