What is the flood discharge capacity? Is there a geography expert who can talk about it in a popular way? Don't be too abstract. I don't understand.

flood discharge capacity generally refers to water conservancy projects such as reservoirs or water conservancy projects. When the rainy season comes, if there is heavy rain, the water in the river will skyrocket and the downstream of the crisis, why not say that the upstream of the crisis is not dangerous and the downstream is dangerous? For example, if the upstream and downstream of the general rainstorm in the basin make rain, then all the water in the upstream will flow to the downstream, and the downstream will also rain. If it falls too much, it will flood the village farmland and even the city. What should I do? Build a dam and a reservoir upstream to stop the water, control the water quantity and let the water pass slowly. For example, the storage capacity of the reservoir is 1 million cubic meters. Usually, when the river is normal, there is only 2 million cubic meters of water in the reservoir without intercepting water. If the gate is closed in rainy season and the water does not come out, it will be piled up in the reservoir, so that the water flow can be controlled by the gate of the reservoir, and the flood will not break out downstream if the water flow is slow. This is flood discharge. How much flood the reservoir can resist refers to the flood discharge capacity. If there is too much upstream water, which exceeds the storage capacity, there is no way out. The reservoir can't hold so much water. The greater the amount of water the reservoir or water conservancy project can intercept, the stronger the flood discharge capacity will be. This explanation is popular enough.