The Torrens Lake was a Mistake
The Torrens River (Karrawirra Parri) is the main river in the city of Adelaide, South Australia, and the surrounding Mount Lofty Ranges. In its heyday, the river provided a much-needed source of water for both the ecosystems and humans inhabiting it. However, as anyone who has visited Adelaide recently will note, the river, and the Torrens Lake that runs through the CBD in particular, has seen better days. The water that runs through it is murky and toxic to consume, the banks are all eroded and/or lined with concrete. What few plants there are lining the riverbanks are often non-native, invasive species and there is so much algal growth in summer that nothing can grow in the water either.
One of the major pollutants in the Torrens Lake, and the rest of the urban catchment, is the prevalence of non-native vegetation. The banks are lined with deciduous trees and grasses, which both result in masses of organic material dumped into the river to decompose. This is a major issue because decomposing material takes up some of the river’s limited biological oxygen, the amount of oxygen dissolved in the water that can be used by plants and animals to breathe. Naturally, the river would be lined with evergreen gumtrees and wattles, so there wouldn’t be a large amount of leaf litter all at once, and what grass was grown, wouldn’t have been mowed. Currently, there are litter traps located just downstream of the St. Peters Billabong to capture the plant matter and remove it from the water, but these have high maintenance costs. There are also some small areas, such as the Billabong and in the Botanic Gardens, where efforts have been made to restore some natural vegetation to the river. However, to truly combat this issue, we need to put aside our historic want for everything to look like Europe, recognise that we are in fact not in Europe, and completely revegetate the riverbank with native, local plants, for the sake of our ecosystems.
Settling a city around a river is inherently problematic for the health of that river. Naturally, the Torrens wouldn’t be confined to the very narrow strip of land that it is today, it would gradually change its course over time, particularly during extreme weather events with large water surges. However, in its current state, the river has nowhere to go and so when there are high storm surges, the riverbanks erode and collapse without being able to adapt. When this happens, revegetation can take years and the ecosystem is vulnerable during this time. It is also dangerous for infrastructure along the riverbank, because if banks collapse, there is a risk of a road or building collapsing with them. In some places, there have been attempts to fix this by simply lining the entire riverbed with concrete, choking out any life that was there for a glorified storm water drain. There have also been less intrusive methods, such as using what is essentially chicken wire to hold the bank in place while plants are still growing, though their effectiveness is yet to be measured. The idealistic solution here is that William Light was an ecologist who took into account the nature of rivers when planning the city, however, this was not the case, so there is little that can be done on a large scale without demolishing and rebuilding large parts of the city away from the river, which would come at great economic and social cost.
The concrete riverbeds are an issue of their own; they speed up the flow of the river and prevent anything from growing. When the river flows quickly, it is harder for plants to stabilise sediment and absorb nutrients, which are critical functions for the health of the river. In order for sediment to be stable, there have to be plants with deep roots to hold the sediment down, and these plants also absorb a lot of the nutrients that, when left alone, cause algal blooms and starve the river of oxygen. Some small areas have been successfully revegetated, including the Billabong and the Botanic Gardens Wetlands, but this is merely the start of recovering the river ecosystem.
It is unrealistic to expect that the Torrens will ever be restored back to its original form, at least not while there’s a city sprawling across it. So, the best solutions are those which help the river adapt to the city around it, while still being a functional ecosystem. This balance between human needs and ecological needs has not always been upheld in the past, but if we want our river to ever be anything more than a mud coloured, stinking disgrace, we are going to have to fundamentally rethink how we treat it.
