The wearing away of the land surface by running water, wind, ice or other geological agents, including such processes as gravitational creep. Geological erosion is natural occurring erosion over long periods of time. Natural erosion can legally reduce the size of a land parcel that adjoins to a body of water.
Source
http://www.mnr.gov.on.ca/en/Business/Water/2ColumnSubPage/STEL02_163778.html