Johnstone River in flood
Photo: Brian Cassey
The North and South Johnstone Rivers rise in the tablelands of the north tropical coast and flow through steep narrow gorges to their junction on the coastal plain at Innisfail. The head waters of the catchments are located in high rainfall areas and the rivers are capable of producing severe flooding, especially in the Innisfail area. The North and South Johnstone Rivers have a combined catchment area of about 1,600 square kilometres at Innisfail.
There is a strong rainfall gradient across the catchment with the heaviest rain generally falling along the eastern side of the catchment around Topaz, Crawfords Lookout and Innisfail. In the western area of the catchment, rainfall totals tend to be significantly less. Heavy localised rainfall along the coastal strip to Crawfords Lookout can cause rapid river rises in the lower Johnstone Rivers around Innisfail and Mourilyan, although larger floods tend to be associated with catchment wide heavy rainfalls.
The Johnstone River delta area can be subject to severe flooding with low lying areas being susceptible. Large areas of agricultural areas can be inundated and residential areas affected. Tides can have a significant impact on the smaller floods but little effect in larger floods.
Severe flooding of the Johnstone River is often associated with tropical cyclones. The two highest floods at Innisfail in recent years occurred in February 1986 and March 1967. The flood records for Innisfail indicate that the highest recorded flood occurred in 1913 and that it was about 1.5 metres higher than the 1986 flood. Historical evidence indicates that the floods in 1893 and 1894 were even higher.