State of the Environment Report 2007
Floodplains are areas of land and water, situated alongside waterways, which are occasionally inundated during floods. They provide a range of ecosystem services. Some wetlands situated on floodplains require regular inundation to replenish them with water, nutrients, seeds for vegetation recruitment, organic matter and to maintain habitat. Fish stocks in waterways also benefit by feeding on organic matter washed from surrounding floodplains. Many settlements and agricultural lands have been established on floodplains because of their fertile soils and proximity of water supplies. For example, both Perth and Mandurah have extended settlements built on the banks of the Swan and Murray rivers respectively. In some areas, residential development on floodplains has contributed to the loss of connectivity between the river, its floodplain and associated wetlands (Figure IW9.1). Other floodplains have been extensively developed are require flood protection (e.g. Carnarvon, Moora, Busselton, Fitzroy Crossing and Northam).
Floodplain isolation occurs when waterways are excessively modified to prevent flooding; damage to crops, stock and infrastructure; and loss of human life. Levee banks on the side of waterways have been built to contain minor and moderate flooding events, thereby reducing the floodplain area that receives regular inundation by flood waters. Some waterways have drain diversions constructed to minimise the effect of floods on nearby towns and agricultural land. Some rivers also have their channels straightened or dredged deeper to hasten water flow through the catchment. All these activities have resulted in floodplains becoming isolated or disconnected from their waterways (i.e. loss of floodplain connectivity).
Dams constructed on major rivers significantly alter the frequency, intensity and duration of floodplain inundation (see 'Altered water regimes'). For example, on lower parts of the Ord River, construction of the Lake Argyle dam has prevented floodwaters from reaching much of its floodplain, even during a one in 100-year flood event. Flows leaving the dam do not generally exceed 1000 cubic metres per second, even though inflows to the dam have exceeded 10 000 cubic metres per second on occasions. This has resulted in a significant loss of downstream river and floodplain ecology. For example, river gum and coolibah trees that rely on flooding are diminishing, leading to weed invasion (Water and Rivers Commission, 2003).
No waterway should be isolated from its floodplain: connectivity is the key to maintaining terrestrial and aquatic biodiversity, and land and water productivity. Removing the hydrological connection of floodplain aquatic ecosystems may eventually result in them becoming terrestrial ecosystems. Floodplain isolation also impacts on river ecosystem health. Lack of organic material and debris washed from floodplains decreases the available food and habitat for fish and aquatic organisms, thereby reducing fish stocks and productivity. Studies show that river production following one day of floodplain inundation was equivalent to 82 years of production under normal conditions (Davies, Bunn & Mosisch, 2001).
It is now appreciated that healthy waterways require adequate flow regimes and hydrological connection with their floodplains. With the exception of pristine waterways and floodplains, it is now impossible to turn back the clock and protect many of the State's floodplains from development.
