A catchment is an area of land, bound by hills or mountains from which all runoff water flows to the same low point. The low point could be a lake, dam, a river or the mouth of a river where it enters the ocean. Rainwater will flow to this lowest point, via creeks, rivers and stormwater systems.

As well as rivers, creeks, lakes and dams, a catchment also includes groundwater, stormwater, wastewater, and water-related infrastructure.  Catchments are connected from top to bottom, so what happens upstream in a catchment has a large influence further down the catchment. 

Human activities across a whole catchment, such as pollution, soil erosion and the spread of weeds, can adversley affect the quality of water and the environment at the bottom of the catchment.

This is why it is important to manage a catchment as a whole, rather than in parts.