There is a seasonal gradient in flow across Melbourne. Out in the rural east, the rivers flow as you would expect; high flows in winter (maximum in August) with dry conditions in January, February and March. As we track west, the seasonal signal is lost and we get similar flows in all months.
Lets look at monthly discharges in seven streams from the Bass River, near Phillip Island, around to the Werribee River.
Flow in the Bass River at Glen Forbes South is strongly seasonal. The graph below shows the flow volume in each month. The horizontal lines are the monthly averages with vertical lines representing volumes for each month as recorded in the years 2000 to 2015.
As we track west, winter flows become less pronounced; see the discharges for the Bunyip River below.
There is a small seasonal signal in Eumemmerring Creek (near Dandenong), but in the streams in urbanised catchments (from east to west – Gardiners, Merri and Kororoit) there is no evidence of seasonality. For these streams, the high flow months are November and February. This is partly driven by some unusually large events, for example, the flood of Feb 2005 stands out on these figures.
For the highly regulated Werribee River, average monthly volumes are higher in summer and spring. The small number of large vertical spikes show this pattern is influenced by a few rare events.
Its difficult to tease out the cause of the differences in seasonality as these streams are affected by gradients in urbanisation, climate, and regulation. Certainly, urbanisation has a strong influence. The impervious surfaces in urban areas will produce runoff all year round; they aren’t influenced by the seasonal wetting and drying that occurs in the rural catchments.
And the consequences? We know that fish, for example, respond to seasonal flow changes to migrate and spawn. But changes in seasonality are probably only a small influence in comparison with the large number of factors affecting the ecology of these streams.