Diversion structure questions
Why is the wall needed?
Most of Lake Rotoiti's problems are caused by nutrients (nitrogen
and phosphorus) entering it from Lake Rotorua. High levels of
nutrients are the main cause of algal blooms in the lakes. More
than 70 percent of Lake Rotoiti's nutrients come from Lake Rotorua
through the Ohau Channel.
Why are so many nutrients getting into the lakes?
Some nitrogen and phosphorus has a natural source. For
example, phosphorus is naturally present in rock. However,
the pressures on the Rotorua lakes are mostly from animal waste
from the intensification of agriculture and growth in lakeside
communities. Many communities dispose of waste to septic tanks. The
nutrients (particularly nitrogen) from this source eventually flow
through groundwater into the lake, boosting algae production.
What does the wall look like?
The wall goes from the lake floor to just above the water
level. It is 1275 metres long, starting south of the Ohau Channel
outlet, which links the two lakes, and extending to Te Akau Point.
The wall is located about 75 metres offshore, running parallel to
State Highway 33.
Are we able to see it?
Yes, the wall rises above the surface of the water and is marked by
lights so that people using the lake, like boaties, can see
How much did it cost and who paid for it?
The wall cost approximately $10 million to build and has been
funded by both Environment Bay of Plenty and central
What are the other options?
Along with the wall, we are doing other things to improve
Lake Rotoiti's water quality. These include sewerage reticulation
in some lakeside communities, upgrading septic tank systems and
restoration work around lake and stream margins.
How long before we see improvements now that the wall is
Scientists are confident that the lake will have fewer
algal blooms within five years.
How long did it take to build?
The wall took approximately 12 months to build.
Will it harm the fish or water birds?
We're confident it won't affect the birds but are still
researching the potential disruption to fish spawning and migration
patterns. It's hard to exactly predict the ecological effects of a
project like this, so we will be monitoring things very closely and
making adjustments when necessary.
Won't it make the Kaituna River's water quality
Scientists say the diversion wall will have little impact
on Kaituna River's ecosystem (e.g. fish and birds), although it
will cause a small increase in the level of nutrients down the
river. Unlike the lake, river systems are not as affected by an
increase of nutrients because the natural flow keeps the water
mixed and moving and algae cannot multiply in the short travel time
to the ocean.
It's worth remembering that water from Lake Rotoiti already flows
into the Kaituna River. In 2004 there were warnings about the
levels of cyanobacteria in the river for the first time. So if we
don't take any action to improve the water quality of the lake and
it gets really bad, the river is likely to be affected even more.
To make sure the river stays protected we are developing a Kaituna
River Management Strategy.
What will the effect on the Maketu Estuary be?
It's likely to have little effect on the Maketu Estuary.
At high tide, just one tenth of the estuary's water volume comes
from the Kaituna River.
We already monitor the estuary and Kaituna River for algal toxins
and will continue to as part of the annual monitoring programme.
When cyanobacterial numbers reach high levels in the Kaituna River,
cyanobacterial toxins will be monitored in Maketu shellfish.
Why not fix the source of the problem?
It is extremely important that we improve Lake Rotorua's
water quality too. A lot of work is being done towards that,
including a multi-million dollar sewerage reticulation programme,
upgrades to Rotorua's sewage treatment, and new rules for