What usually leads to channel erosion becoming unstable?

Channel erosion often becomes unstable due to increased flows or changes in upstream sediment loads. When there is an increase in water flow, it can result from factors such as heavy rainfall or upstream construction activities, leading to more water entering the channel than it can handle. This increased velocity can erode the channel banks and bed more aggressively, destabilizing the channel.

Additionally, changes in sediment load can occur due to land use changes, deforestation, or mining activities upstream. If the sediment load entering the channel increases, it can lead to sediment deposition in certain areas while causing erosion in others, further destabilizing the riverbanks. This dynamic interaction between water flow and sediment transport is crucial, as it determines the stability of the channel and how it responds to environmental changes.

The other choices do not typically lead to instability in channel erosion. Strong vegetation growth can actually help stabilize banks by holding soil together with roots. Consistent rainfall over a long period could lead to some erosion, but it is the intensity or rapid nature of flow changes and sediment dynamics that most directly contribute to instability. Draining water away from the area usually reduces the water levels in streams and can prevent erosion rather than encourage it.