November and December have seen some very significant rainfalls throughout the catchment, from 84mm in November and 212mm in December around Boreen Point, and up to 107mm and 269mm around Tewantin. These have highlighted the different features of the flows of the creeks and the river. December tests in particular were taken immediately following 48mm overnight and 66mm a week earlier.
Salinity decreased throughout, many locations showed almost fresh water, obviously reflecting the recent rainwater floating above any brackish or saline layer. Groundwater also becomes more acidic following rain, measured as a decrease in pH reading from the usual 7-8 values. In the fresh water tributaries, pH in the Upper Noosa River fell to 4.2, Eenie Creek to 4.89, and others had pH values of 5 – 6.
Turbidity shows the greatest effects from rainfall, and probably the most informative for investigating silt solids pollution entering the river. In the Upper Noosa River turbidity rose from 1.2 in November to 6.0 in December, a small rise only, because the river runs through natural sand country. In contrast Kin Kin Creek turbidity rose from 1 to a very high 81 in December, and at Kinaba from 1.5 to 82.8, therefore much erosion is still occurring in that creek. At Boreen Point this incoming silt becomes partly diluted in the volume of Lake Cootharabah, where turbidity rose from 8 to 14.4. Cooloothin Creek turbidity also increased, from 2.8 to 12.9, so some silt also enters the lake from that source.
Further down river from Lake Cootharabah, above Tronsons Drain the river turbidity measured 16.7 in November rising to 25 in December, but immediately downstream of the drain (the outfall of Ringtail Creek) turbidity went the other way dropping from 91.5 to just 15.7, salinity was also greatly reduced, and this effluent water was very brown. Here the explanation is that the rain was flushing this stream which mainly flows through forest, and its brown colour is due to high tannin content from decaying vegetation rather than from silt erosion. This is accords with its increased acidity, the pH dropped from the normal 7.5 to 6.55.
Similar results were found in Wooroi Creek, both salinity and pH were lower, the effluent water was again quite brown, but the turbidity dropped at all three sites along the stream, at the creek mouth turbidity dropped from 24.5 to 15.5. Again, the stream was being flushed by the rain, adding tannin to the water, but with little erosion occurring. Photos below show the brown effluents flowing from Tronsons and Wooroi Creeks.
In the main estuary section of the river downstream of the ferry similar changes have been observed, however invariably they are a much smaller magnitude because the small flows from each tributary are rapidly diluted in the larger water volume and flow rate of the river. Some typical turbidity results comparing November and December were: Makepeace Island 14.7 dropped to 14.3
Lake Doonella (Marina) 11.5 dropped to 8.2
Weyba Creek 4.0 dropped to 3.3.
These tests demonstrate the well known fact that the main source of silt in the river appears to be from Kin Kin Creek, with a small contribution from Cooloothin Creek, and much silt is then washed from Lake Cootharabah downstream to the river mouth. Smaller creeks flowing through well vegetated country contribute fresh water containing tannin as a natural biological byproduct. Tests recorded in the main river are relatively insensitive for identifying sources of pollution
All nutrient residues in the water have remained very low, and within the same ranges as previous tests.
Thanks to the Waterwatch team members who have contributed to all the above work.