Why can flood-control reservoirs no longer prevent flooding after they become full?

Once flood-control reservoirs reach their maximum capacity, they can no longer serve their intended function of mitigating flood risks. At this point, the reservoir loses its ability to store incoming water, meaning that all water entering the system must be immediately released downstream.1

Because the reservoir is full, it can no longer provide the "lag effect" that typically helps reduce peak discharge rates during high-water events. Consequently, the structure ceases to provide additional protection, and the volume of water passing through the dam effectively matches the volume of water flowing into it. This limitation highlights the inherent challenges in relying solely on engineered structures to manage the complex hydrology of a major river like the Mississippi, as these systems can be overwhelmed by extreme weather events.213