Report: TR 2015/13R
Author: B Vant
The colour and clarity of a waterbody is determined by its water quality—in particular by the concentrations of (1) algae, (2) suspended silts and clays, and (3) dissolved coloured substances such as tannins. The different ways in which these materials interact with beams of underwater light all combine to give a waterbody its characteristic appearance. For example, high levels of algae in lakes give the water a murky, greenish appearance, while high levels of silt and clay mean that many rivers appear brown and muddy during a flood.
In this report, Waikato Regional Council scientist Bill Vant used the existing records from the routine water quality monitoring sites on the Waikato and Waipa Rivers to determine how the various constituents affected the clarity of the water in the different parts of the river system. We know that the physical environment differs in the Waikato hydrolakes, in the un-impounded Waipa, and in the Lower Waikato downstream of its confluence with the Waipa. So it’s reasonable to expect that the clarity of the water will also differ in these areas.
At the head of the Waikato River—Taupo Gates—water clarity is very high: it’s usually possible to see more than 7 metres through the water. By Lake Ohakuri the clarity has fallen to 2-to-3 metres, and by Tuakau it’s down to 0.5 metres. Clarity also declines down the length of the Waipa River; but near the head of the river it’s just 1-to-2 metres, falling to 0.5 metres at Whatawhata.
On average, algae are responsible for about half of the drop in clarity in the Waikato River between Taupo Gates and Ngaruawahia. In the Waipa River, suspended silts and clays are responsible for most of the drop in clarity. Downstream of Ngaruawahia suspended silts are about twice as important as algae.
Average levels of algae in the Waikato River have declined over the past decade or so. But water clarity hasn’t improved much, and then only in certain parts of the river (e.g. at Narrows). At Lake Ohakuri, clarity was actually poorer during the period when algal levels were lower than they had been previously. The means that silts and clays can often mask the effects of algae on water clarity.
Even so, we can make some approximate predictions about how water clarity is likely to change if silt and algal levels in the river are managed. The report describes a method for making predictions like this.
Note: This report was originally published in May 2015. This revised version corrects minor errors in tables 2, 3 and 4 (and the accompanying text) of the original report. These corrections have made little or no difference to the interpretation or conclusions.
|2||Current water clarity and constituent concentrations|
|3||Contribution of constituents|
|5||Sources of constituents|
|6||Summary and conclusions|
|Appendix 1: Phytoplankton from the shallow floodplain lakes|
|Appendix 2: Long term changes in nitrogen and phosphorus|