Southern View: March 09, 2023

Thursday March 9 2023 3
Rock riffle to help movement of fish
A NATURALISED rock riffle
being installed in the Ōtākaro-
Avon River near Mona Vale
Homestead will help native and
sport fish easily move to and
from the river’s upper catchment.
The rock riffle has been
specifically designed for New
Zealand’s fish species and will be
similar to those seen in nature.
A naturalised rock riffle
is a long ramp with a gentle
gradient. Rocks are strategically
placed to allow for a wide
range of flow conditions to
cater to the variety of weak and
strong swimming fish species.
Larger emergent boulders are
also placed in the waterway in
clusters to create resting places
for fish and spawning habitat for
invertebrates.
It will be installed over the
existing weir. Work began last
month and is expected to take
six months to complete.
Said city council spokesman
Gavin Hutchison: “Barriers to
movement in waterways have
been identified as a significant
threat to both native and sports
fish in New Zealand.”
More than 76 per cent of
New Zealand’s living native
freshwater fish are classified
as threatened with or at risk
of extinction, Department of
Conservation figures show.
“The remediation of this
weir will allow access to
approximately 9km of waterway
upstream of the weir and
increase migration, diversity
and abundance of ika in the
upper river’s catchment,” said
Hutchison.
“Limited passage of brown
trout occurs at this weir but
likely not full passage. In
addition, the weir is a barrier to
many of our native ika (fish) who
are poor swimmers, in particular
inanga (whitebait), bluegill bully
and lamprey, which all have
conservation status.
“The native fish are unable
to swim or jump through the
barrier and that’s why we find
very few native species in the
waterways upstream.”
More than half of New
Zealand’s native fish species
travel back and forth from
freshwater to the sea to complete
their life cycle. The new rock
riffle will enable easy access to
upstream waterways.
It will be similar to a rock riffle
previously installed near the
Antigua Boatsheds.
The history of the Mona Vale
weir dates back to the 1860s
when the Deans family leased
six acres of their land adjoining
the Avon River loop (including
water rights) to William Derisley
Wood for the construction of a
flour mill.
In November 1860 Wood
called for tenders from ditchers
and excavators to cut the
mill race across the river, and
construction began around
FLOW: A naturalised rock
riffle being installed in
the Ōtākaro-Avon River
near Mona Vale will allow
fish such as brown trout
access to 9km of waterways
upstream.
PHOTO: NEWSLINE
February 1861.
In order for the mill race
to obtain a sufficient flow of
water to turn the mill’s wheel,
Wood was required to install a
weir within the channel of the
Avon River loop as a means of
controlling the water flow.
Extensive repairs to the
original timber weir were carried
out in 1939 and the existing
concrete weir was renewed by the
Christchurch Drainage Board in
the mid-1980s.
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