Groups will thus be able to share and compare data to get a picture of possible changes in the health of the whole catchment of the stream along its length. When it rains, the water drains naturally to the lowest point on the land, forming into small creeks that feed into larger streams and rivers as they run downhill. In this way water can drain into a stream from an area that is often many square kilometres in area. Natural features such as ridges, hills or mountains form the boundaries of a catchment. From these high points, all run-off water flows downhill to the lowest point.
Catchments vary in size and make-up.
Large ones are bordered by mountain ranges and include hundreds of small sub-catchments. Each sub-catchment may itself be bordered by low hills and ridges and drained by each smaller creek or gully. What happens in all of these smaller catchments and streams will affect the well-being of the main river. So, an occurrence at one spot along the river will be influenced by what has happened upstream, and will itself affect water quality downstream. Before development, the natural catchment.