Effect on Aquatic Ecosystems


Aquatic ecology surveys have shown the lake and river environment supports a total of 31 benthic invertebrate taxa including mayfly, stonefly and caddisfly. Periphyton cover is described as light, the heaviest population being dominated by the blue green algae Phormidium in the lower reaches of the Matiri river where mild organic enrichment is suggested.

Fish populations in the lake and Matiri river have been surveyed extensively. Koaro, long finned eels and upland bully have all been recorded although Koaro are shown to be a lacustrine population and are consequently not found below the lake.

Lake Matiri and the reaches of the river immediately below the lake are free of introduced trout.

Turbid water at number 1 outlet during low flow.

It has been noted during site visits that even during low flow conditions the water has a slightly turbid appearance in the reach immediately downstream of the lake. This is thought to reflect the active erosion and frequent land slipping of the steep mud stone slopes along the middle reaches of the river.

During construction of the weirs the lake level will be significantly lowered, this is expected to have only minor impact because the lake level will be lowered for a short period.

During normal operation of the scheme the lake level will be more stable than would naturally occur and operation of the scheme is not expected to adversely effect waterfowl breeding and feeding areas around the lake edges or benthic invertebrates and native fish populations inhabiting the lake.

Naturally occurring 1000 litres per sec river flow

Normal operation of the scheme will reduce the water flow in the river between the weir and the tailrace, however a minimum flow of 1000 litres per second will be maintained at all times. The overall reduction in flow will result in a reduction in the total area of periphyton and invertebrate habitat in this reach. It may also cause some reduction in the quality of the remaining habitat during periods of low summer flow as water velocities reduce, temperatures increase and periphyton biomass increases.

These adverse effects will be mitigated to some extent by naturally occurring flood events augmented by periodic flushing flows to be provided via the sluice gate located in the main weir.

Naturally occurring lake levels for 1998 which was an average flow year.

The passage of eels and koaro is catered for by maintaining the minimum river flow at all times and a fish pass designed into the main weir. The fish pass operation will be aided by a holding pool at the base of the weir. Suitable screens will be provided to prevent the ingestion of fish to the penstock.

The stranding of fish due to rapidly varying flows below the tailrace will be mitigated by ramping the generation level up and down over a 10 minute period thus reducing the rate of flow change. This effect will be further moderated by inflow from the west branch some 800m downstream of the tailrace and will become progressively less pronounced in the lower river.