What does the long profile of a river illustrate?

The long profile of a river depicts the gradient or slope of the river from its source to its mouth, illustrating how elevation changes along its course. This profile typically shows a decrease in height as the river flows downstream, starting from the higher elevations at the source and gradually reaching the lower elevations near the mouth, where it meets another body of water, such as an ocean or lake. The long profile is crucial for understanding various river processes, including erosion and sediment transport, as these processes are influenced by the elevation and gradient of the river.

While options related to water speed, river width, and vegetation may provide valuable information about different characteristics of the river, they do not specifically convey the essential information about how elevation decreases from source to mouth, which is the fundamental aspect illustrated by the river's long profile.