What part of the stream valley is covered by water during a flood?

During a flood, the area of the stream valley that is covered by water is typically the floodplain, which includes the low-lying regions adjacent to the stream or river. This area is designed to absorb excess water when the stream overflows its banks. Additionally, the water may cover parts of the channel and surrounding terrain that are normally dry. Floodwaters can extend further into adjacent areas, depending on the severity of the flood.