Types of flows and benefits
It’s not simply the amount of water flowing in a river that is important. It is the entire environmental flow ‘regime’ that matters, including the volume, timing, duration, frequency and quality of flows that are provided. Like the natural flow of rivers, different combinations of these provide a different range of benefits for each ecosystem.
Freshes through summer, for instance, help to maintain or improve water quality, while spring flooding replenishes a river channel and provides soil and nutrients for floodplains, as well as being vital for the breeding success of water birds and native fish.
Flow components describe the different parts of a flow regime relevant to an ecosystem. They are characterised by their size, frequency, timing and duration. Table 1 outlines the types of flows and benefits typical to a Victorian river system.
|Type of flow
|Cease to flow
||This may lead to either total or partial drying of the river channel, depending on the specifics of the system.
||The period of no discernible flow in a river. A drying cycle is important for some ecological processes. It also has the benefit of reducing numbers of exotic pest species, such as European Carp.
||Low flows generally provide a continuous flow through the channel. This may either maintain the flow above a ’cease to flow‘, or provide habitat as a change from ’high flows’.
||Connects in-stream habitats and can have benefits for other system users, such as providing water for livestock.
||Small or short duration peak flow events. These are flows that exceed the base flow and last for at least several days.
Freshes are a key contributor to the variability of flow regimes, providing short pulses in flow.
|Helps to maintain or improve water quality and prevents algal blooms from occurring.
||Persistent increases in the seasonal base flows that remain within the channel.
High flows do not fill the channel to ’bankfull’.
|Allows for fish migration and enhances recreational fishing opportunities.
||Flows of sufficient size to reach the top of the river bank with little flow spilling onto the floodplain.
||An important trigger for fish breeding. Helps with sediment movement and bank maintenance. Also enhances recreational opportunities.
||Flows greater than ’bankfull‘, resulting in inundation of the adjacent floodplain habitats.
Overbank flows are critical for a range of ecological factors, including floodplain productivity.
To prevent damage to people and property, environmental water is not released to provide overbank flows on private land.
|Maintains floodplain and wetland connectivity, stimulates fish and bird breeding and enhances recreational opportunities
Regulated rivers are those in which water is stored in large reservoirs and released for irrigation and urban water supply. Regulated rivers can have flow patterns opposite to those which would naturally occur. This is because water is stored in reservoirs in winter (when river flows would naturally be high) and then released to irrigators in summer (when rivers would naturally be drier).
When allocations from environmental water entitlements are put into a waterway to protect or improve its health, the aim is to emulate natural flow components.