Opouteke River

The Opouteke River drains from predominantly exotic forestry (pine forest), through a small area of pasture before reaching the Mangakahia River. The only pastoral farming land use upstream of the sampling site is sheep and beef farming, although there are dairy farms further downstream. Acidic volcanic rocks make up the underlying geology.

The Opouteke River results for 2005-2006 are summarised in the table below including the median, range and percentage of sampling occasions that comply with relevant guidelines for the 12 sampling occasions. Medians shown in red are outside the recommended guidelines.

Parameter Median Range % comply with guideline
Temperature (deg. cel.) 15.4 11.3 - 23.5 -
Dissolved oxygen (% Sat.) 109.1 101.8 - 116.5 42
Conductivity (mSm) 13.1 11.5 - 14.9
Water clarity (m) 1.88 0.55 - 3.65 92
Turbidity (NTU) 3.0 0.4 - 10 67
E. coli (n/100mL) 111 23 - 336 55
Dissolved reactive phosphorus (mg/L) 0.016 0.002 - 0.072 42
Total phosphorus (mg/L) 0.025 0.007 - 0.079 58
Ammoniacal nitrogen (mg/L) 0.005 0.005 - 0.07 92
Total nitrogen (mg/L) 0.162 0.054 - 0.427 100
pH 7.8 7 - 8.3 42


Similarly to last year dissolved oxygen concentrations (% saturation) and phosphorus levels are often elevated in Opouteke River. However phosphorus levels were not as high in 2005-2006 as they have been in the past, with approximately half of the results for both total and dissolved reactive phosphorus within their respective trigger value. The trend analysis on the ten years data available for Opouteke River shows that dissolved reactive and total phosphorus levels are decreasing.

Unlike other sites on the network, the source of this phosphorous is probably not agricultural run-off, as nitrogen and bacterial levels are low. More research is required to investigate the problem further, partly because the source of the high phosphorus levels is unclear and also they are exhibiting an unusual oscillating pattern.

Like many of the other sites in the RWQMN there is a significant increasing trend in pH in Opouteke River (i.e. trend analysis indicates that this site is becoming more alkaline). This could be a detrimental trend at this site, as pH is often above the 7.2 to 7.8 trigger value range.