What does flow regulation do to rivers, estuaries and fish?
“I love a sunburnt country,
a land of sweeping plains,
of ragged mountain ranges,
of drought and flooding rains”
Anyone who has ever read this famous poem penned by Dorothea MacKellar in 1906 instantly understands that she was describing Australia. Australia’s aquatic ecosystems have adapted to not only withstand long periods of low flows punctuated by flood events, but to actually require them for healthy function.
Research has shown that many of our fish species are triggered to spawn, move and feed by flood events, with some travelling hundreds of kilometres in response to cues provided by flows. Changes in water depth and water quality parameters such as temperature and dissolved oxygen are all thought to be important in providing these triggers to our native fish.
Floods are also thought to increase survival of fish at larval and juvenile lifestages, creating huge surges in productivity as floodwaters are spilled out onto the floodplain, where it mixes with nutrients, creating algal blooms and population explosions of invertebrates that young fish feed on.
High flows are also important for maintaining the mix of aquatic vegetation species present, whose density and abundance can be controlled by periodic flushes. And it is not just the aquatic ecosystems which rely on changing flows; nearby land-based communities can be driven by them as well. Huge river redgum forests lining banks of the Murray River rely on periodic flooding, which in turn provide temporary habitat and ideal spawning sites for native fish during high flows. In estuaries, saltmarsh and mangrove habitats can also require tidal inundation to remain healthy.
How to humans impact on flows and what impact does this have on fish?
Unfortunately humans haven’t been so effective in adapting to the ‘boom and bust’ nature so typical of water availability in Australia. Our solution has been to build large dams and water storage facilities to provide water security during periods of low flows. These dams can impact on downstream waterways, and the fish which live in them, by altering the quality of water downstream, and when flows are released.
Whilst natural floods are usually nutrient and oxygen-rich, artificial flows released from dams are typically cold, and low in oxygen and nutrients. This is caused by creation of cold layers of water at the bottom of deep waterbodies which can be completely lacking in oxygen and nutrients, as the sun’s energy only warms the shallower surface waters (see the figure below for a better understanding of this). This not only produces habitat areas within the impoundments which are unsuitable for fish, but can also result in the release of water which is low in oxygen into the waterway downstream, as many impoundments release waters from bottom outlets. This oxygen-poor, dramatically colder water can impact on the health of downstream waterways for kilometres, reducing productivity right through the food chain, and causing reduced growth, feeding and breeding of native fish species.
Water held back by dams and harvested into water storages is released downstream to be used for irrigation and creation of hydro-electric power, and often these industries require that water be released at a different time of year to when natural flood events usually take place. Consequently, these flows occur at the wrong time of year to support ecological functions. Water releases are often made for irrigation purposes during summer when stream temperatures are normally high. This is also when many species breed, and if optimal temperatures are not reached their sexual development and successful spawning may not occur. The feeding activity and metabolic rate of fish is also influenced by water temperature, and colder temperatures can result in reduced feeding activity, slower growth rates, and possibly also stunted growth. These sort of impacts have been observed in a number of catchments throughout Australia, such as downstream of Tallowa Dam on the Shoalhaven River (prior to installation of a top-shot spillway) and possibly also the Dumaresq and Gwydir rivers in northern New South Wales (though more data is required to confirm this).
What is being done to minimise human impacts on flows and fish?
Managers of water infrastructure (dams and weirs) can help to minimise their impact by providing ‘environmental flows’, whereby water is deliberately released into a river, wetland or coastal zone to maintain ecosystems and their benefits. The provision of environmental flows is a relatively new concept. Environmental flows can be provided in two ways: either through release of water from dams, or through preventing downstream water users from abstracting water (abstraction is where water is removed from a river for irrigation or some other purpose). Often it is important that both of these approaches are applied at once, to ensure that water released from a dam for environmental benefit is not “soaked up” by human users further downstream.
Whilst environmental flows are helpful, they often aren’t as effective as natural flows from an ecological perspective. Because it is recognised that dam releases are often quite different in character to natural high flows, water managers often try to combine environmental flow releases with natural high flow events, so the two mix, and create an even larger flow. Water infrastructure managers are also installing ‘Multi-level offtake’ structures on dam walls, which allow water to be released downstream from anywhere in the water column, making it more similar to natural flows.
Harris, G. 2006, A dying shame – Australian coastal freshwater lakes’, paper prepared for the 2006 Australian State of the Environment Committee, Department of the Environment and Heritage, Canberra, http://www.deh.gov.au/soe/2006/emerging/lakes/index.html