Why can't alluvial plains be covered by rivers and become rivers?

Alluvial plain, as its name implies, is formed by the river flowing over the river channel and deposited around it with quicksand. After the flood, the river may be diverted to low-lying areas, forming a new channel-this is the case in the history of many diversions in the lower reaches of the Yellow River, or it may remain on the original channel. The old river channel left after diversion may turn into a lake, which is how Taihu Lake is formed.