Erosion – The wearing away and removal of material by turning it into a solution or by attrition, abrasion or hydraulic action
Types of erosion:
Solution: The sides of the river dissolve into the water.
Attrition: Wearing down the river bed by bouncing into the sides of the river and breaking off particles.
Abrasion: Wearing down the river bed by grinding, rubbing, scratching and scuffing like sandpaper.
Hydraulic Action: The force of water against riverbanks compresses air into cracks, which expands and fractures the rock over time.
To remember that erosion is attrition, abrasion, hydraulic action and turning material into a solution, learn the following story:
Eros, he rose up upon (erosion) a hydrant and started acting on it (hydraulic action). First, he tried being a donkey and made a braying (abrasion) sound that gave everyone a headache, so the solution (solution) was two headache pills. They did stay, however, to see Eros do a trick on (attrition) the hydrant.
The erosive power of a river over time is perhaps best illustrated by land formations such as America's Grand Canyon. Water running off the Rocky Mountains formed the mighty Colorado River, which cut into the rock, sometimes changing direction as obstacles were encountered and then worn away. This action carved out the canyon over millions of years.