What is meant by the term 'base level' in river systems?

The term 'base level' refers to the lowest point to which a river can erode. This concept is crucial in understanding river dynamics and the processes of erosion and sediment deposition. The base level is often represented by the level of the body of water into which the river flows, such as the ocean or a lake. Once a river reaches its base level, it can no longer erode its channel vertically; instead, it may widen and meander or deposit sediment. This understanding is important because it helps explain the formation of river valleys, deltas, and the overall landscape changes caused by river activity.

In the context of this question, the other definitions do not accurately capture the meaning of 'base level.' The highest point a river can reach, the average flow rate, and the elevation at the river’s source pertain to different aspects of river dynamics and topography, but they do not reflect the specific concept of base level regarding erosion.