The state of our rivers
NIWA scientists, in partnership with the Ministry for the Environment and with assistance from 15 regional councils, have conducted the most extensive study ever of water quality in New Zealand rivers.
Effective management of water resources requires up-to-date information both on their current state and on changes through time. The objective of our study was to provide water quality information for New Zealand rivers at different levels of detail and scale – from individual rivers to New Zealand as a whole. To achieve this, we used the River Environment Classification (REC) (www.niwa.co.nz/ncwr/rec/).
By aggregating sites according to REC classes, we were able to produce clear pictures of current water quality at both regional and national level. Water quality was associated with a combination of natural factors (e.g., climate, topography, geology) and human factors (e.g., land use). For example, streams draining high-rainfall areas had better water quality than streams in drier climates, streams draining lower elevation areas had poorer water quality than streams draining higher areas, and streams draining pastoral catchments had poorer water quality than those in natural catchments.
An analysis of trends in the data from 1996 to 2002 showed that the level of contamination by waste piped into waterways (point-source pollution) had dropped, showing that efforts by farmers, factory owners, and local authorities to improve management of direct waste discharges is paying off. However, when we examined non-point sourced contamination we found national-scale evidence of increased concentrations of nutrients (e.g., phosphates), particularly in streams draining agricultural land. In contrast to the clear spatial patterns in water quality status for particular REC classes, we were not able to identify river classes of specific management concern with respect to trends. This outcome reflected the under-representation of reference sites in some classes of river.
Reference sites (comparatively unmodified upstream catchments) are needed to distinguish variability that is due to large-scale climate factors from that associated with smaller-scale land use factors. A key message from this study for resource managers is that monitoring networks must incorporate appropriate reference sites, so that underlying drivers of water quality trends can be clearly identified.
Mike Scarsbrook Megan Linwood, Ministry for the Environment ([email protected])
For further details, including supporting publications, contact Megan Linwood.