Erosion is a natural process in which the surface of the Earth – soil, rock and other surface material - is worn down and moved from one location to another by erosive agents. Erosive agents are water, ice (glaciers), snow, wind, plants and animals. Humans can also be forces for erosion, and can erode surfaces 10-15 times faster than natural processes.
According to Joe Magner, hydrologist at the University of Minnesota, there are four sediment pathways and processes of erosion relating to waterways:
Surface erosion or denudation
The erosion that occurs in relatively flat small scale areas such as a crop field or urban yard. Soil can erode as a sheet or rill but water then converges at larger scales into a gully.
Gully and hillslope erosion
The slope becomes steeper and gully formation can occur which creates clear visible erosion. In humid areas groundwater seepage can contribute to erosion by reducing the critical shear strength of the soil.
Landsides and mass wasting occurs in very steep slopes. This erosion is driven by changes in soil water content and changes in bank angle or even earthquakes.
Stream channel erosion is a natural phenomenon; however, excessive scour can occur when runoff magnitude changes and channels enlarge to accommodate changes in land use or climate.