When a river dries
Results of an extensive study on the Selwyn River in Canterbury will help water managers setting acceptable limits on water abstraction.
The 18-month study by NIWA scientists involved comprehensive electric fishing surveys at 10 sites on the river to quantify the effect of stream drying on native fish communities. The river dries naturally and ‘disappears’ in its middle reaches, getting to its lowest in late summer or early autumn, but this process can be exacerbated by groundwater abstraction.
As expected, the density and diversity of native fishes was lower in reaches that dried seasonally. Even occasional, short-term drying events had a marked and prolonged impact on the fish community. Species varied greatly in their vulnerability to drying and recovery of the fish communities in the intermittent (middle) reaches depended upon the distance to and direction of permanent water.
In the upper river, many Canterbury galaxias, upland bullies, and brown trout migrated upstream to permanent water as the river dried from the bottom up. The upper intermittent reaches dried frequently and most fish species were slow to re-establish themselves there.
In the lower river, the stream dried from the top down, but as the fish did not appear to migrate downstream to permanent water, many would probably have died. The river dried less frequently here than in the upper intermittent reaches, and upland bullies and eels recolonised these areas from ‘refuges’ downstream relatively quickly.