The term "riverbed" refers to the bottom or channel of a river, which is typically composed of sand, gravel, rocks, or boulders. It's where water flows during times of high flow and it can change over time due to natural processes such as erosion, deposition, and sedimentation. The characteristics of a riverbed play an important role in determining the behavior of the river, including its depth, velocity, and ability to support aquatic life or human activities like fishing, swimming, or boating.