From
the history of European development, we expect gradual changes in flow of the
Mangakotukutuku Stream occurred between 1900 and 1960, with gradual development
and intensification of farming in the watershed. But the 1960’s arguably
presented a far more drastic alteration of water and sediment flows for the
Mangakotukutuku Stream (in addition to physical infilling of gullies). For
Sandford Park, one potential scenario is a massive increase in sediment supply
during the 1960’s, as a result of earthworks runoff and increased stream power
(eroding both natural bank material plus infilled material upstream of the
park). Sandford Park may have been an active depositional zone for this
material, with a wider floodplain and perhaps a lower gradient at its
confluence with the Waikato River (cf. upstream and tributaries). Subsequent
stabilisation of both the earthworks and infill in the catchment would have
reduced sediment supply while flows remained high, or possibly increased with
impervious area. Sediment starvation would then allow channel incision and bank
erosion through the 1960’s deposits in Sandford Park. Many new dams were built
on the Waikato River between 1940 and 1970, cutting off much of the bedload.
The resulting scouring of the Waikato River bed has potentially lowered the
outlet of Mangakotukutuku Stream, reducing the capacity of this area as a
depositional zone.
The
recent erosion through Sandford Park (since 2000) indicates some tipping point
was reached that has destabilised the channel. But this probably reflects more
than just the most recent urban development and associated stormwater. In
restoring the stream, we are faced with the question of what to restore it to.
The state of Mangakotukutuku of 1970-1990 may have been as artificial and
transitory as the severe bank erosion we see now. We do know however, that any
increase in peak flow will only make the problem worse and take us further from
achieving stability. Council and landowner practices that reduce channel
resistance (e.g. removing log jams, channel realignment) also exacerbate the
problem by increasing the erosive power of the stream.
Enhancing fish habitat in Sandford Park
Tonkin & Taylor restoration report:
The stream care group commissioned a report on approaches to stabilising banks and creating cover to enhance native fish habitat in Sandford Park
Read the report (902kb); Appendices available on request (7973kb)
NIWA habitat enhancement study:
NIWA is planning an experiment on habitat enhancement
in a small tributary of the Mangakotukutuku Stream running through
Sandford Park. The experiment targets koura and banded kokopu, which
have been found in low numbers in the stream and which are known to
seek out wood-related habitat. Wood will be added to the stream in
such a way as to improve conditions for these species (with the
expectation that their populations will increase) while avoiding undue
risk of debris and flooding problems in stormflows. A long-term goal
of this research is to develop guidelines for stream restorers.
Vegetation restoration activities
Learn about some experiences of group members in restoring gully vegetation:
