The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. What do lakes, rapids, waterfalls, and terraces indicate about the stream gradient and base level? Why do canyons form in some places and valleys in others? (A)

Lakes, rapids, waterfalls, and terraces are all features that are commonly found along a stream. Each of these features indicates a different stream gradient and base level. 1. Lakes typically form in areas with gentle gradients and low base levels, as the water flow is slow, allowing for sediment deposition and the creation of a natural reservoir. 2. Rapids are indicative of an area with a steep gradient, where the water flow is fast, causing turbulence and the formation of whitewater. Rapids generally form in areas with relatively high base levels or at the junction of two streams with different gradient and flow characteristics. 3. Waterfalls occur in areas with steep gradients and significant changes in the underlying geology, causing a sudden drop in the stream's base level. This abrupt change in elevation results in a dramatic fall of water from a higher level to a lower level, creating a waterfall. 4. Terraces form along stream channels when the base level changes over time, either due to tectonic activity, glacial retreat, or other factors. These step-like formations are a record of the stream's historical base levels and the stream's adjustment to these changes.

Canyons and valleys are both erosional landforms formed by the action of a stream. However, they differ in their formation processes, which can be attributed to factors like geology, climate, and tectonic activity. 1. Canyons typically form in arid or semi-arid regions where the stream gradient is relatively steep or the underlying rock is resistant to erosion. Canyons are formed by the process of vertical erosion, which deepens the stream channel through the cutting action of the water flow and sediment transport. Canyons are characterized by their steep walls and narrow, V-shaped profiles. 2. Valleys, on the other hand, form in more humid and temperate regions with more gentle gradients where the stream has a more meandering path. In addition to vertical erosion, lateral erosion is a dominant process in valley formation, resulting in a widening of the stream channel. Valleys usually have broader, U-shaped profiles and are often filled with sediment deposited by the stream. In conclusion, lakes, rapids, waterfalls, and terraces all indicate different stream gradients and base levels, while canyons and valleys are formed due to varying geological, climatic, and tectonic factors influencing the stream's erosional processes.