Although permafrost can extend hundreds of feet deep, it is only the top section that concerns builders.
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Answer:
Explanation:
Rivers have three sections or courses: the upper course, middle course and lower course. The upper course of a river typically sees much erosion of bedrock in order to achieve the equilibrium mentioned above.
The upper course is also at the highest altitude as this is where the headwaters of the river originate. Rivers in the upper course erode down vertically which often creates a steep channel profile in what becomes the river valley.
The middle course sees a river at a slightly lower altitude than the upper course, but is still maintains a sense of trying to acquire equilibrium in flow and shape. The discharge and velocity of the river still allows the water to erode the banks and chart a course of least resistance through lateral erosion.
The lower course is unique as this is the stage in which a river has reached sea level. Here we find features not found in the middle and upper courses like floodplains and river deltas that have been formed by the sediment deposits of rocks and dirt gathered from upstream.
