Understanding the Water Resources of the Cardrona River, Central Otago

Abstract

The Cardrona River runs in a north-east direction from the Crown Range to the Clutha River Mata-Au at Albert Town, near Wanaka, in Central Otago. There are water resource concerns in the catchment regarding a lower reach that dries up during the hottest and driest part of the year, February – March, uncertainty about surface-groundwater connectivity in parts of the river, and the river’s ecological health specifically regarding stream temperature. In 2021 new water abstraction limits are legislated to be enforced on the river and underlying aquifers, creating a political environment of interest in increasing the knowledge of the water resources of the Cardrona. The aim of this research was to provide a better understanding of the water resources in the Cardrona River. A field sampling programme was conducted from March 2016 to April 2017, and included continuous flow and water temperature data collection at key sites down the river and discrete water chemistry sampling that included sampling for ionic and molecular chemistry on a monthly basis, stable water isotopes seasonally, and radioactive radon (222Rn) during winter and summer flow.

Flow through the valley was found to be mostly consistent throughout the sampling period, while there was temporal variation observed in the final 10 km of the Cardrona River, where it flows over an alluvial fan to the Clutha Mata-Au. Generally, the river lost volume over the fan, but it did so differently over the different months, indicating a dynamic combination of hydrologic pathways. The chemical analysis indicated that there was some gain from groundwater in the lower part of the valley, where the river was considered in other research to be neutral with regard to gain or loss from subsurface flow. Furthermore, it appeared that the final reach of the river, downstream of the dry riverbed, was not primarily sourced from the Clutha Mata-Au or underflow from upstream in the Cardrona, but from groundwater, particularly from late January to late February. Stream temperature was also variable down the river, and there was no simple relationship of water temperature with flow at most of the sites. The two locations that did have a correlation between flow and stream temperature during summer conditions were at the top of the valley and 2 km from the confluence with the Clutha Mata-Au. Both of these sites were among the coolest sampled and the sites with the warmest water temperature tended to have the poorest association with water volume.