An Assessment of the Hydrology of the Pool Burn and the Management Opportunities for Restoring Stream Function

New Zealand’s freshwaters have become increasingly scrutinised for reduced flows and deteriorating water quality, prompting more stringent management by regional authorities, and enforcement of environmental flow limits across all water bodies. A catchment of this nature is the Pool Burn, a channelised tributary of the Manuherikia River in Central Otago, New Zealand. Low flows have become characteristic of the Pool Burn during the warmer seasons, which has been exacerbated by the controlled releases from water storage systems and the increased abstraction for irrigation purposes. The primary objective of the study was to characterise the current state of the Pool Burn, through observing longitudinal and seasonal variations of key hydrologic parameters. Where water abstraction for irrigation and preservation of native fish species serve as two main values within the catchment, the outcome of the study aims to inform water users of potential management options to ensure water availability both spatially and seasonally. An empirical approach was adopted for this study, where hydrological, hydraulic, and water quality data were collected at seven sites established down a 20 km stretch of the Pool Burn, between October 2016–October 2017. A stable flow regime was evident for the irrigation season (October–April) for the Pool Burn, dominated by relatively low flows, varying between 0.04 – 0.14 m3 s-1 across all monitoring sites. Flow variability did increase between May–October, ranging between 0.06 – 0.57 m3 s- 1. Morphological responses to the low flow variability of the Pool Burn were most obvious through changes in depth compared to changes in width, which is characteristic of channelised streams, where reaches have become constrained to well-defined banks with relatively uniform beds. Temperatures exceeded the upper tolerance threshold (>19 ºC) at the lowest sampling site, which was attributed to the shallow, wide channel morphology, and lack of riparian planting. Furthermore, the absence of riparian planting and the stable flow regime were factors causing high phosphate concentrations (14 – 41 g P L-1) during the warmer seasons. However, a decrease in phosphate and an increase in nitrate and ammonium was observed during the wetter seasons, attributed to the greater export of nutrients to and from the stream. The effects of a regulated flow regime and abstraction for irrigation were most obvious in the downstream sections of the catchment. Therefore, proposed flow limits for the Manuherikia catchment were predicted for the Pool Burn, which showed that sufficient flows were not sustained if minimum flows (2 – 3 m3 s-1) for the Manuherikia were experienced. Alternative management options are required for the Pool Burn, to reach flow volumes suitable for providing habitat for the native fish species and improve the recreational use of the stream. Ensuring regular releases from the water storage infrastructure is implemented, as well as reducing all water takes proportionally will improve conditions observed at the downstream section of the catchment. Additionally, introducing a 2 – 4 m riparian buffer strip along the entire length of the stream will provide shading and habitat for aquatic species, and also behave as a nutrient sink, alleviating nutrient enrichment and periphyton growth in the stream. The results from the current research have highlighted the importance of adopting an integrated approach to understand the state of heavily influenced, agricultural streams, where a simple minimum flow approach will not ensure connectivity or improve restoration attempts