The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. What factors determine the position of a local base level? What is the ultimate base level, and why? (C)

A local base level is an elevation or level below which a stream or river cannot erode its bed. It represents a temporary equilibrium or balance between erosion and deposition in a particular area.

The ultimate base level refers to the lowest elevation that a stream or river can possibly erode its bed to. For most rivers, the ultimate base level is equal to the sea level because rivers typically flow into the oceans.

Several factors determine the position of a local base level, including: 1. Topography: The shape and steepness of the landscape affect the elevation of a river or stream's base level. In mountainous areas, for instance, the base level may be much higher than in flatter regions due to the overall topography. 2. Human intervention: Human activities like dam construction can create artificial local base levels by raising the elevation of the riverbed and thus limiting the stream's ability to erode further. 3. Geology: The type and resistance of underlying rock layers can influence the base level elevation. Streams that flow over highly resistant rocks may have a higher base level than those that erode easily eroded rocks. 4. Tectonic activity: Movement of the Earth's crust can raise or lower the local base level. In areas with active tectonic uplift, the base level may rise as the land is pushed upward, while subsiding areas could cause the base level to drop.

The importance of the ultimate base level lies in setting the lowest elevation that a river can erode its bed. It effectively limits the erosion potential of any given stream or river because once this level is reached, there will be no more potential energy available for further erosion. Streams and rivers can only deposit sediment at this point, creating equilibrium in the landscape. The ultimate base level also influences the formation of various geological features, such as deltas and floodplains, as streams and rivers slow down when they reach this level and start depositing sediment.