Woody waterway debris

Article and images by Gordon Agnew, waterways technical officer, Sunshine Coast Council

With increased development occurring on the coast, some of our local creeks are losing their natural habitat value and are now functioning as modified drains. This is particularly true for many of the creeks and channels within urban residential areas. In comparison, unmodified natural creeks are places of exploration and childhood memories and contain overgrown branches, old logs, impenetrable reeds, fallen trees, leaves and sticks all of which provide essential food and habitat for our aquatic wildlife.

Much of this material comes from the vegetation beside the creek known as the riparian zone. The riparian zone is the land alongside creeks, wetlands, gullies and rivers. This diverse area of land is often the most fertile part of the landscape. Riparian vegetation is the primary contributor of food and energy for our creeks and rivers in the form of flowers, leaves, bark, branches and logs. This organic material is vital for the functioning of aquatic ecosystems but is often seen as unsightly debris that should be removed. However, it provides an important food source for native fish and other aquatic fauna such as macroinvertebrates (bugs without backbones) and should be left in place. Fallen trees and logs also stabilise the creek bank and reduce the speed of the water preventing erosion.

The amount of organic material found in a creek is often linked to the age of the vegetation in the riparian zone. Modified urban creeks with younger vegetation and limited overhanging branches tend to be in poor condition, have limited shading and contain very little natural material such as large logs.

Consequently, they have limited aquatic diversity and often favour pest fish species and weeds. In contrast, natural unmodified creeks with towering trees and thick native riparian vegetation tend to be in very good condition and provide greater habitat complexity and a more diverse suite of native fauna. Often, these towering trees adjacent to the creek eventually fall in creating important fisheries habitat.

Regarding fisheries habitat, if you ask a fisherman where to catch the best fish, they’ll often describe a location where there’s a big fallen tree with lots of overhanging branches and structure in the water. Structural diversity within a creek in the form of logs and fallen trees is generally an indicator of a good fishing spot. These old logs and trees are referred to as snags. Snags provide fish with shelter, shade, resting areas and protection from predators. For example, the endangered Mary River cod are thought to require hollow logs for spawning (Simpson and Jackson, 1996) while other species such as Australian Bass prefer to live around snags (Marshall, 1979).

Unlike a modified creek, natural creeks provide a diversity of stream habitats for macroinvertebrates platypus and fish. Logs and branches also provide lookout sites for predatory birds such as cormorants and resting sites for water dragons and turtles.

So next time you visit a creek and see a fallen tree or log, think about leaving it there rather than removing it. Better still, contact your local natural resource group and get involved in riparian planting activities or plant native trees along the creeks of your own property. Your native aquatic wildlife will be so thankful.

If you would like advice on how to improve the health of your creek contact council's waterways and catchment management team via customer contact on 5475 7272.

References

• Marshall, R. 1979, ‘A bit about bass’, Modern Fishing, Sept, pp.25-9
• Simpson, R. & Jackson, P. 1996, ‘The Mary River cod recovery plan, Project no. ESP 505’, Report for the Australian Nature Conservation Agency, Canberra.