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
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
||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