Melbourne's rivers and creeks once exhibited the natural features and ecology of coastal waterways. Most have, however, been heavily modified. While the larger Yarra and Maribyrnong river systems are well known, there are also many tributaries and other creek systems that flow from their smaller upper reaches into larger channels, into estuaries and finally into Port Phillip Bay. In the upper and mid-reaches, the aquatic fauna is entirely freshwater, whereas in the estuary more salt-tolerant species are joined by many transient marine species, which move in and out from time to time.
Stream types range from small upland rocky boulder creeks (Olinda Creek, for example), which flow from the slopes of Mount Dandenong, to the slower flowing lowland sections of the Yarra River, which are dominated by deeper, meandering channels and banks lined with river red gums. There are also many short, coastal streams (for example, Balcombe Creek on the Mornington Peninsula), which flow only a short distance directly into Port Phillip Bay.
The ecology of the upper reaches is dominated by the processing of organic matter that enters the stream from the catchment and surrounding vegetation. This organic matter, together with photosynthesis occurring in algae and stream plants, provides the nutrient base for the stream food chain. These nutrients are processed by a variety of stream invertebrates - first by microbes, which begin the decomposition, and then by other invertebrates. The invertebrates can in turn be eaten by aquatic vertebrates such as fish, birds and platypuses.
The interface between the river and the surrounding catchment is the riparian zone. The importance of vegetation in this zone cannot be overemphasised. It acts as a buffer to the surrounding activities, and filters inputs and runoff into the stream. Much of the important instream habitat, in the form of woody debris, comes from this zone, as do the other organic nutrient inputs. Root systems also hold the riverbanks together, preventing erosion and hence additional sediment transport. Continuous vegetation along waterways also provides a corridor for terrestrial animals and birds. Unfortunately much of Melbourne's streamside flora has been removed.
As with most urban waterways, many of Melbourne's rivers and creeks have been subject to pollution, often in environmentally damaging ways. The clearing of catchments and asphalting of urban areas have led to changes in water flows through increases in run-off rates. Some of Melbourne's rivers also have dams, which store water for urban supply. This has reduced some stream flows and, together with use of water during summer, can lead to extremely low flows and water-quality problems. Road-building and catchment-clearing are also responsible for rises in sediment loads and turbidities, and increased nutrients have led to higher algal counts. Occasional fish kills and other water-quality problems have reduced the number and diversity of both fish and aquatic invertebrates in some areas. Habitat-removal has been widespread, with some waterways, such as Dandenong Creek, having been channelled and reduced to concrete drains for large sections. Road-making, including the use of creek corridors to house freeways, has meant that some channel sections have been relocated, piped or otherwise heavily modified. Such modifications provide very little habitat for aquatic fauna. Large woody debris, which provides important habitat and spawning sites for many species, has been widely removed from many urban streams. This makes many of these streams very bare and uniform, favouring many of the introduced fish species, which have less specific habitat requirements and wide water-quality tolerances. These species now dominate many of Melbourne's urban waterways.
Growing environmental interest and a realisation of the importance and potential of Melbourne's creeks and rivers have increased activities aimed at rehabilitation. Driven by community interest and participation, together with waterway and State Government agencies, a range of projects have worked to improve Melbourne's watercourses, water quality, catchment environments and aquatic fauna. Such projects include rubbish removal, replanting of riparian and catchment vegetation, and the provision of structures to improve fish passage. Many creeks have their own community action groups (such as Mullum Mullum Creek and Merri Creek).
As well as the flowing rivers and creeks, there are many other off-stream waters such as wetlands and billabongs, which also provide important habitats for other aquatic species, including the threatened dwarf galaxias. Unfortunately many of these habitats have also been altered, drained or had their connections with the river blocked. These areas have traditionally also acted as filtering mechanisms for water entering the river and have delayed much of the flash flooding that has been exacerbated by urbanisation. Further restoration needs to be undertaken to ensure the future ecological functioning of Melbourne's rivers and creeks and to restore their native flora and fauna.