A watershed (or catchment, as commonly used internationally) is the area of land drained by a river or creek system. Higher land, mountains, or hills bound a watershed. If human activity is restricted or not permitted in the watershed, it is known as a protected watershed . Human activities and nature both affect water quality in many ways, including point source and non-point source pollution. Apropriate treatment to provide the right water quality for an intended use depends on the type and degree of contamination. If the contamination contains nutrients used by aquatic biological organisms, it may cause eutrophication in a downstream source.