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Stream restoration

Implications of historical changes for stream restoration

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

Habitat restoration report:
The stream care group commissioned a report by Tonkin & Taylor 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)

Habitat enhancement study:
NIWA carried out an experiment on habitat enhancement in a small tributary of the Mangakotukutuku Stream running through Sandford Park. The experiment targeted 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 was added to the stream in such a way as to improve conditions for these species 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. See Newsletter #3 for progress details.

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 improves access to upstream habitat for migrating juvenile fish such as giant and banded kokopu.
New fish pass at Peacockes Rd

Wetland construction 

A wetland construction plan developed for the group by Tonkin & Taylor for a site at the end of Sandford Park near Peacockes Rd is providing conditions suitable for mudfish which once occurred in the catchment. Spring water previously discharging into the gutter on Peacockes Rd was diverted and chanelled into a sealed and bunded area planted which was with native wetland vegetation. The water levels are controlled in such a way to create conditions suitable for mudfish which need periodic wet and dry conditions to survive. The original plans includes a plan view, a cross section view, and a plan for the outlet structure. Contstruction of the wetland was funded by the Waikato River Clean Up Trust.

Project completion reports

Vegetation restoration activities

Learn about some experiences of group members in restoring gully vegetation:

Stream inspections