A catchment is a natural boundary of a waterway, defined by the shape of the landscape. Rainfall in a specific catchment generates rainfall-runoff, that eventually flows into the estuary downstream through a network of streams, creeks and rivers, as well as through underlying groundwater and stormwater drains in built-up areas.
Not all rainfall-runoff ends up in estuaries. Water can infiltrate land and be stored in aquifers, or surface runoff can be stored in reservoirs. Soil type, land use and the slope of the landscape influence this. The balance between the amount of rain that falls on a catchment area and the responding volume of water flowing into an estuary downstream is complex.
This surface water runoff and groundwater from a catchment may contain sediments, nutrients and other pollutants. The condition or health of an estuary is influenced to a certain extent by the condition of its catchment.
The size of a catchment area is important in determining the:
- amount of water that flows into an estuary
- amount of pollutants, such as sediment and nutrients, that enter an estuary
- physical shape of an estuary
- condition of the opening to the ocean.
Some catchments are small, and others cover thousands of hectares. Larger catchments are a collective of many smaller sub-catchments. Sub-catchments typically surround a single small stream or creek, and a larger catchment surrounds multiple streams and creeks, sometimes hundreds! Upland catchment areas are inland away from the coast and are often hilly and steep, and floodplain catchment areas are flat areas of land next to a stream, river or estuary.