This low point could be a lake, dam, river or the mouth of the river where it enters the ocean. A catchment also includes groundwater, storm water, wastewater, and water-related infrastructure.
All water in the catchment is connected
The diagram below illustrates how a catchment forms a main river.
Catchments are connected by this flow of water, meaning what happens to water upstream or on-farm influences water quality downstream. Even temporary streams and channels have an effect on water quality downstream because they typically make up a large volume of water flowing over land.
Ground and surface water are connected
It is important to note that groundwater and surface water are also connected and may interact. This interaction is shown in the diagram below.
- Surface water can reach ground water when it filters through soils to recharge groundwater.
- Groundwater can reach surface water when solid rock layers prevent water infiltration downwards. This creates shallow groundwater flows which can reach the surfaces as a spring. Groundwater can also reach surface water if the elevation of the groundwater next to a surface water body is higher than the water level in the stream.
This means that nutrients in groundwater have the potential to enter surface water and vice versa.
How different water bodies react to excess nutrients, sediment and bacteria
The amount of nutrients, sediments and bacteria reaching a water body will depend on catchment features such as land use and practices, rainfall amounts and patterns, soil types and slope, and vegetation cover that intercept rainfall. Different types of water bodies react differently to excess nutrients, sediments and bacteria. Once in a water body the responses to nutrients (and sediments) differ between lakes and rivers.
In a river, the flow determines the effect of excess nutrients on plant growth. The flow regime includes the volume of water, how fast the water is moving and when it flows. Rainfall is unevenly distributed across catchments and through seasons, so river flow is dynamic.
During high flows, like floods, river water is often fast moving. These flows flush out sediments and nuisance plant growth. The load may end up wash out to sea, deposited onto land or transferred into the next catchment. However, in catchments with lakes and estuaries, high flows can deposit surplus nutrients in these waterbodies that can cause issues at other times of the year.
During low flows, rivers water is often slow moving e.g. include dry summer periods or periods of demand for irrigation. The lower flow speed reduces the ability for the river to transport sediment and phosphorus which can settle on the stream bed.
Problems that can occur under low flow include sediment build up and nuisance plant growth because flows cannot flush the channel. The lower volume also concentrates nutrients which poses greater environmental risk.
During low flows, nitrogen enriched groundwater can become the dominant source of flow in rivers increasing nitrogen concentrations.
Low flows can also reduce habitat availability and increase temperature and oxygen stress on aquatic life.
Lakes are especially vulnerable because rivers deposit nutrients or sediment loads in them where they accumulate over time.
Shallow lakes are sensitive to excess nutrients and sediments because wind and waves tend to stir up sediment more readily. This reduces clarity and shades out high value plants on the lake bottom, therefore resulting in loss of aquatic habitat for main species and a decline in water quality.