Mangakotukutuku Stream Care Group Inc.
|
Mangakotukutuku Stream Care Group Inc. |
|
|---|
|
|||||
Implications of historical changes for stream restorationFrom
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
Habitat restoration report: |
|||||
| Helping out with fish passage: Work on the stream culvert under Peacockes Road has helped improve passage for migrating native fish thanks to funding from Waikato Catchment Ecological Enhancement Trust and the Hamilton City Council. The work was carried out by a contractor, and involved constructing a concrete ramp to remove the high velocity drops at the culvert pipe and the downstream apron, and cementing of stones in place to create resting areas for fish. The fish pass will improve access to upstream habitat for migrating juvenile fish such as giant and banded kokopu. |
 |