Mangakahia River at Twin Bridges

The Mangakahia River originates from Matarua Forest and flows southwest until it reaches the Wairua River. There are two sites on the Mangakahia River in the network; one at Twin Bridges and the other at Titoki Bridge in the lower reaches. The Twin Bridges lie at the confluence of the Awarua and Mangakahia Rivers. The sampling site is directly upstream of the confluence, on the Mangakahia River. Located in a reserve area, Twin Bridges is a popular swimming spot and picnic area for tourists and locals alike. The upstream catchment is a mix of plantation forestry, and beef and sheep farming, with a volcanic acidic underlying geology.

The 2005-2006 results at Twin Bridges on the Mangakahia 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.) 15.8 11.7 - 24.2 -
Dissolved oxygen (% Sat.) 107.1 102.4 - 115.9 42
Conductivity (mSm) 11.0 8.7 - 19.3  
Water clarity (m) 1.64 0.52 - 2.73 92
Turbidity (NTU) 3.3 0.5 - 9.1 75
E. coli (n/100mL) 135 52 - 354 45
Dissolved reactive phosphorus (mg/L) 0.024 0.002 - 0.102 25
Total phosphorus (mg/L) 0.046 0.00 - 0.143 42
Ammoniacal nitrogen (mg/L) 0.005 0.005 - 0.19 83
Total nitrogen (mg/L) 0.252 0.061 - 0.409 100
pH 7.7 7 - 8.2 50


Mangakahia River had the second highest temperature of all the RWQMN sites recorded in 2005-2006 on 15 February 2006 of 24.2°C (Waiarohia Stream at Second Ave had the highest). It is generally thought freshwater ecosystems, particularly fish and macroinvertebrate communities are likely to be detrimentally effected if water temperatures exceed 25°C. It is likely that temperature exceeds this at Twin Bridges at certain times of the day in summer months.

The greatest concern with water quality at the Twin Bridges site is the elevated phosphorus levels that are occurring all year around. Both the median amount of dissolved reactive and total phosphorus for 2005-2006 exceeded guideline trigger values, with less than half of the sampling occasions for both parameters being within guidelines. There is also a significant increasing trend in dissolved reactive phosphorus.

The median of dissolved oxygen percentage saturation and bacterial levels (E. coli) were also above the recommended levels, but the elevated E. coli results tend to occur in conjunction with increased flows and the high dissolved oxygen levels occur in summer when photosynthesis rates by plants are greater. Overall, the Twin Bridges site represents a moderately impacted site, which could easily be improved with some riparian planting and fencing.