How can spillway discharge affect flooding downstream from a dam?

Spillway discharge can impact downstream areas primarily through the rate of water release. Ideally, the rate of floodwater discharge should not significantly exceed the levels experienced before the dam was constructed. If the discharge rate increases, it can lead to flooding problems downstream.12

However, dams generally help mitigate this risk by reducing the peak discharge rate. This occurs due to a "lag effect," where the reservoir temporarily stores a portion of the floodwater as it passes through, effectively smoothing out the flow.12

Additionally, the physical nature of the discharge itself poses a risk. Water flowing over a spillway can reach extremely high speeds, creating highly turbulent conditions upon entering the riverbed. If this energy is not properly controlled and dissipated—often through specialized structures like stilling basins or "kickers"—the resulting turbulence can cause significant erosion to the riverbed and potentially damage the dam structure itself.12