Flow regime requirements and the biological effectiveness of habitat‐based minimum flow assessments for six rivers

Abstract

Sustaining instream values when there is demand for out‐of‐stream water use is challenging for water resource managers and often there is considerable debate about the methods used to assess flow requirements. Recommendations for flow regime requirements for benthic invertebrates, trout and indigenous fish were made using instream habitat analyses in six New Zealand rivers. We review the results of studies that were carried out to examine the response of aquatic communities to the flow changes. Although the biological data may not be scientifically rigorous in all cases, the weight of evidence from the various sources indicates that in 5 out of the 6 cases, the biological response and the retention of desired instream values, was achieved using the habitat‐based methods for setting flows. Indeed, there were increases in trout and benthic invertebrate abundance and changes to the invertebrate community structure in the rivers with successful outcomes. In some cases, flows and flow variability were far from natural, yet excellent trout and invertebrate communities were sustained by the modified flow regimes. High (i.e., flushing) flows were beneficial for cleansing fine sediment deposits and filamentous algae in one river where this regime was recommended and in one case uncontrolled spring floods were necessary to open the river mouth and allow recruitment of diadromous fish species.

Keywords:

• Minimum flow assessment
• instream habitat
• flow regime