The Mangere River is a low-lying sluggish tributary to the Wairua River, flowing through a mostly intensive agricultural catchment. The river begins as the Mangere Stream, which flows east out of the Pukenui forest near Whangarei, becoming a river on the flats before joining the Wairoa River just west of Kokopu. For the most part, soft sedimentary rocks make up the underlying geology with small pockets of riparian vegetation remaining along the river.
The 2005-2006 results for the Mangere River 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||10.6 - 20|
|Dissolved oxygen (mg/L)||8.6||5.7 - 10.2||92|
|Dissolved oxygen (% Sat.)||84.8||62.8 - 91.7||0|
|Conductivity (mSm)||17.2||15.9 - 24.5|
|Water clarity (m)||1.11||0.45 - 1.87||75|
|Turbidity (NTU)||5.2||3 - 19.5||58|
|E. coli (n/100mL)||898||461 - 4352||0|
|Dissolved reactive phosphorus (mg/L)||0.120||0.069 - 0.292||0|
|Total phosphorus (mg/L)||0.172||0.101 - 0.591||0|
|Ammoniacal nitrogen (mg/L)||0.050||0.005 - 0.23||17|
|Total nitrogen (mg/L)||1.179||0.416 - 2.23||17|
|pH||7.2||6.6 - 7.6||50|
The Mangere River is the most impacted river in the regional network with the worst water quality of the 21 sites monitored. Nutrient and bacterial levels are consistently elevated above guideline values throughout the year. A already high E. coli median of 898 per 100mL for 2005-2006, may only get worse as a long term trend as been detected that suggests that E. coli counts are increasing at a rate of 86/100mL/year.
However there have also been several trends detected that are a positive result for the highly degraded Mangere River. This includes decreasing ammoniacal nitrogen, decreasing nitrate/nitrite nitrogen, increasing pH, increasing water clarity, decreasing temperature, and decreasing total nitrogen. If these trends of improving water quality continue, there is a chance that water quality in the Mangere will noticeably improve.