Our Lake Cootharaba team in particular have reported some very interesting observations following heavy rain and storms recorded on January 16/17 and 18/19, when Boreen Point received 64 and 69 mm respectively, but rainfall thereafter was much lower through into February.  Water tests were made immediately afterwards on January 19 and then routinely in February.   Salinity and pH of the water both always drop following fresh rainfall, whereas turbidity reveals useful information on river bank erosion and subsequent water pollution.

In the Upper Noosa  River turbidity and pH both fell a little from dilution by the fresh rainwater, turbidity at 1 NTU remained at that level through January and February.

In contrast Kin Kin Creek, Kinaba and Cooloothin Creek in January produced the highest turbidity readings on NICA records, 166, 129 and 103 NTU respectively. They also showed larger falls in both pH and salinity.  Then the extreme turbidity readings had all dropped dramatically by February. These changes suggest that heavy rain was very localised to the catchments to the west of Lake Cootharaba, and caused significant erosion and addition of silt to those two streams, but the silt was quickly washed out from the streams over the following few weeks of calmer weather.  The Upper Noosa River to the north-east, flowing through sandy country, was not affected by the rain.

In the lake, measured at Boreen Point jetty, pH and salinity both fell a little from the added rainwater, but turbidity also dropped from a high of 63 in December to just 10 NTU, and remained at that level through to February.  This is unexpected, considering the high turbidity in two of the streams feeding into the lake, and even reaching as far as Kinaba.  Perhaps an explanation for this is that the turbidity in the water settled quickly upon entering the lake if it was not being disturbed by wind or storms.

The very high turbidity in the western tributaries provides a graphic illustration of the continuing serious erosion of their banks, as is well known at some locations upstream especially in Kin Kin Creek.  More extensive monitoring at a number of locations along those streams is used to identify the sources.

Further downstream Tronsons Canal produced a similar flush of turbid water in January but had settled back to normal conditions in February of relatively clear but brown stained water caused by tannin from decaying vegetation.  In the lower river and tributary creeks (Wooroi, Cranks, Eenie Creeks and Lake Cooroibah) where turbidity had been high following October/November rain, turbidity has dropped throughout, resulting in clearer water in the urban areas though showing the usual month to month variability.

There has been concern about possibly extremely polluted water at the Lake Cootharabah Canoe Launching Ramp, where black sludgy water has been apparent at the lake shore.  This was referred to Noosa Council for investigation, and they have advised that the black sludge appears to be primarily caused by an accumulation of fine carbonaceous material suspended by wave action in the water.  It is highly probably that this originated from the bushfires in 2019/20, has been driven across the lake surface by the prevailing S-SE winds, and been locked into the small foreshore pocket at the launching ramp.  The Environmental Health Team investigation concluded that the water along this beach, while visually unattractive, presents no health hazards to users of the canoe facility.  Our thanks to Noosa Council staff for promptly investigating the complaint.

We have continued collecting water samples for nutrient testing by Noosa Council.  Since August nitrates, phosphates, and ammonia have been within detectable limits.  Organic nitrogen is present at all test sites, but within permitted WQO limits.  Slight increases in nutrients following rain events appear to be dissipating, and there is no nutrient leaching from agricultural, domestic, or industrial environments.

Thanks to the Waterwatch team members who have contributed to all the above work.

By Bruce McConkey