Appendix 9 Aquatic Ecology Review Memorandum - Greater ...

28. From aerial photographs in GIS it also appears that the ‘dam overflow channel/realigned
stream’ falls at least partly within the catchment of another small catchment south of the
main unnamed catchment. Clarification is required as to whether this is the case or not; if it
is, additional information on the effects on this catchment may be required.
Reduced flow, flow variability and magnitude of flushing flows
29. The application has two proposed scheme operations, ‘summer’ (November to April) and
‘winter’ (May to October). Under the summer operation, all natural flow in the unnamed
tributary will be diverted around the dam (ie, left in the stream) unless flow exceeds 25 L/s.
If the tributary is flowing greater than 25 L/s, harvesting of water into the dam will occur
(ie, in summer, a residual minimum flow of whichever is the lowest of the natural stream
flow or 25 L/s will be maintained in the diversion channel and unnamed tributary). During
the months May to October (inclusive), water is proposed to be harvested at any time that
flow in the Waipoua River exceeds 1,000 L/s, including at times when the natural flow in
the un-named tributary is less than 25 L/s (ie, in winter, the 25 L/s residual flow will not
necessarily be maintained).
30. Regardless of the operation (summer and winter), when water is harvested (diverted) into
the dam there will be less water volume flowing through the diversion channel and into the
unnamed tributary below the dam. Additionally, harvesting will impact on the variability of
flow experienced downstream, including flushing flows (as water is harvested under these
higher flows). These effects will be most severe in the unnamed tributary but will also be
exhibited, in decreasing amounts, in the Wakamoekau Creek and the Waipoua River. These
effects have not been well documented in the AEE although some are briefly discussed in
Cheal (2013).
31. Potential ecological effects from the harvesting (ie, reduced flow and reduced flow
variability) that should have been discussed in regards to the ecological values of the
unnamed tributary, the Wakamoekau Creek and the Waipoua River include:
a) A reduction in aquatic habitat available;
b) A reduction in the dilution of contaminants (eg, nutrients);
c) An increase in water temperature and subsequent decrease in concentrations of
dissolved oxygen;
d) A loss of connectivity with marginal (edge) stream habitat and the ‘ecosystem
services’ these areas provide;
e) An increase in periphyton and macrophyte growths due to increased flow stability;
f) A reduction in flushing/removal of periphyton and macrophyte growths; and,
g) A reduction in flushing/removal of streambed sediment.
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