What is considered the Ultimate Base Level?

The Ultimate Base Level refers to the theoretical maximum point of erosion for all streams within a given area, and this level is typically defined by the elevation of sea level. When a stream erodes landscape materials, it can only wear down to a certain point; that point is usually determined by the level of the body of water it eventually flows into. Since oceans and seas represent the ultimate repository for all surface waters, sea level is considered the ultimate base level. This means that no matter how much a river or stream tries to erode further inland or upward, it cannot effectively do so beyond the level of sea water, which serves as the ultimate end point for sediment deposition and water flow.

In contrast, options like the highest point of a plateau, local stream levels, and temporary lake levels do not serve as ultimate limits for erosion or deposition processes on a large scale, as they can be altered by geological events, sediment buildup, or human activities, unlike the relatively stable and global nature of sea level.