What is a physical reason why river basins are naturally vulnerable to flooding?

One reason is due to a lack of vegetation in a river basin. Vegetation acts as a barrier to surface runoff, therefore it reduces overland flow. If vegetation is limited, water will quickly enter river channels, which reduces the lag time time. Lag time is defined as the time between peak rainfall and peak flooding. The shorter the lag time, the more intense the flooding, therefore a river channel's bankfull capacity is more likely to be exceeded.