Restoration of Ohinemuri River

 

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The bird call heard on this page: Tui

Restoration of the Ohinemuri River Channel at Waihi a community initiative

Recommendations for revegetation using indigenous plants (by Roger MacGibbon Indigenous Forest Specialist)

Introduction

The Ohinemuri River is fed by several tributaries arising in the hills surrounding Waihi township, it flows past the southern edge of Waihi and from there flows through the Karangahake Gorge and eventually into the Waihou River on the Hauraki Rains.

In the vicinity of Waihi the Ohinemuri flows along a narrow, modified flow channel that is virtually completely denuded of all indigenous vegetation. In most places around the town's periphery the river is grazed to the waters edge and where it is not the banks are covered with a mixture of rank exotic grasses, scattered clumps of willows and extensive patches of exotic weeds and shrubs, the most abundant of which would be blackberry, gorse and broom.

The stream bed in this area is highly degraded with considerable stream bank erosion especially where the banks are grazed and pasture covered. The water carries a high sediment load which is likely to be largely the result of the bank erosion and runoff from adjacent farms.

The river is in considerable need of restoration and as such is a particularly suitable focus for a community based environmental initiative, especially because of its close proximity to Waihi township. It rates highly in its need for environmental restoration and, once restored, would offer substantial recreational and educational benefits to the local community.

Physical description

The river is prone to reasonably frequent flooding where the water level can rise by as much as 2 to 2.5 metres. The most extensive bank erosion occurs in this zone.

The water's edge throughout section 1 and section 2 is characterised by severe topsoil slumping (see photo 2) and after each flood large grassed topsoil "islands" are washed into the river channel and downstream.

Local indigenous riverside vegetation and species recommendations

A survey of the indigenous vegetation associated with streams in the Waihi area was carried out to determine the most appropriate native species to be planted along the Ohinemuri, and the list below of recommended native species for the restoration of the river be made up predominantly of those occurring in the local area. Some of the more eroded portions of the Ohinemuri within the flood zone will, however, require further native species that are not represented In the more stable natural river systems observed In the area. These additional species have been added to the lists below. The lists below have been divided into different planting zones and this information; should be used by those organising the plantings. Some species occur in more than one list.

Zone 1: Upper river bank (occur above normal flood zone) and terrace flats

A. Colonising species (ie. can be planted out In the open)

bulletAristotelia serrata (wineberry) #
bulletCarmichaelia aligera (native broom)
bulletCoprosma robusta (karamu) #
bulletCordyline australis (cabbage tree) #
bulletCoriaria arborea (tutu) - on steep eroding banks #
bulletHebe stricta (koromiko) #
bulletKunzea ericoides (kanuka) - in drier areas #
bulletLeptospermum scoparium (manuka) - in wetter areas #
bulletMelicytus ramiflorus (mahoe) #
bulletMyrsine australis (red matipo)
bulletPtttosporum eugenioides (lemonwood) #
bulletPittosporum tenuifolium (kohuhu) #
bulletPseudopanax arboreus (five finger) #
bulletWeinmannia racemosa (kamahi)

B. Secondary species (that can be planted once or where there is some protective cover)

bulletBrachyglottis repanda (rangiora)
bulletCoprosma grandifotia (kanono)
bulletCoprosma ludda (karamu)
bulletCyathodes juniperina (mingimingi)
bulletGeniostoma rupestre (hangehange)
bulletMacropiper exceteum (kawakawa)
bulletOlearia rani
bulletPomaderris kumeraho (kumerahou)
bulletPseudopanax crassifolius (lancewood)
bulletSchefflera digitata (pate)

C. Tree species that can be planted once the colonisers have established some weed control and cover

bulletAgathis australis (kauri)
bulletBeilschmiedia tawa (tawa)
bulletDacrycarpus dacrydioides (kahikatea)
bulletDysoxylum spectabile (kohekohe)
bulletHedycarya arborea (pigeonwood)
bulletKnightia excelsa (rewarewa)
bulletPhyllodadus trichomanoides (tanekaha)
bulletPodocarpus totara (totara)
bulletWeinmannia silvicola (tawhero or towai)

Zone 2: Species suitable for the the zone between normal water level and seasonal high flood level

D. Colonising species

bulletCarex dipsacea
bulletCarex secta
bulletCoprosma robusta (karamu) - only on the upper half of the flood zone
bulletCoriaria arborea (tutu) - on eroded banks on the upper half of the flood zone
bulletCortaderia fulvida (toetoe)
bulletJuncus gregiflorus (bulrush)
bulletLeptospermum seoparium (manuka) - only on the upper half of the flood zone
bulletPhormium tenax (flax)

Recommended revegetation strategy

Ideally, the area to be planted should extend from the water's edge (at normal flow levels) up the banks on both sides of the river and at least 5 metres over the top of each bank (see diagram below) so as to stabilise the bank and reduce erosion to a minimum.

The aim of all planting should be to establish a solid cover of indigenous vegetation over the planting area as rapidly as possible. The transitional period between when the exotic weed cover is removed to enable planting and when the native trees and shrubs are planted and become established needs to be minimised. For this reason and to reduce plant loss from flooding, planting of each section should be carried out in spring.

The size of the area chosen for planting each year should not be greater than can effectively be maintained over the ensuing 3 years. It is wise to estimate the resources and funds that are likely to be available for post-planting maintenance first and from this determine the number of plants that should be planted each year. The opposing banks of the river should be planted at the same time and the entire planting area from river level to bank top of each bank should be clothed in a single planting session.

The area chosen for planting each year should be permanently fenced to exclude livestock before planting is commenced. Electric fencing is not recommended.

Planting specifics

1.Site preparation

Invasive exotic weeds, especially blackberry and gorse, should be sprayed prior to planting and once spray dieback is obvious the dying growth should be cut back to near ground level to enable new plants to be planted. Where grasses predominate 1 metre diameter planting circles should be sprayed. If the grass consists of extensive dead seed heads then the planting circles should first be cut to near ground level and then sprayed to allow the best uptake of spray. Except for the problem weeds such as blackberry the banks should not be blanket sprayed because the root systems of the grasses will be needed to anchor the slopes until the roots of the native plants take hold.

Roundup, or its equivalent, is the only safe general purpose spray to use where native trees and shrubs are to be planted. This chemical is effective at killing grasses and in most areas is effective on blackberry and gorse although repeat sprayings may be necessary.

Planting circles should be located approximately 1.5 metres apart (and certainly no further apart than 2 metres) and care should be taken when spraying the circles not to create straight planting lines along or up and down the slope.

All trees including oaks, willows and any other species of reasonable size should remain on the river banks. Their root systems play a vital role in holding the upper banks together, and only once the native trees are very well established should removal of any of the exotic trees be contemplated.

2. Plant quality

All of the plants listed in schedule B and C above should be well grown in PB3 or PB5 containers and be of a minimum height of 35 cm (for upright species) at the time of planting. Those plants listed in schedule A with a # symbol after them could be planted from Hilson root-trainers as long as the minimum plant height is 30cm. The other species listed in schedule A are best planted from PB2 or PB3 containers with the minimum plant height being 35 cm.

With the exception of the tutu, all species listed in schedule D should be planted from PB2 containers. The plants that are to be planted in the flood zone must withstand regular flooding and so the bigger the root system at the time of planting the more likely it is that those plants will survive their first spring and summer. Once their root systems have had time to establish the species to be used in this zone have considerable tolerance of the actions of flooding. The tutu in this zone are to be planted into the exposed banks and so are best planted from root-trainers so that minimum disturbance of the banks is caused. If at all possible, the plants used in this project should be sourced from parent material that occurs naturally in the Waihi area. Where this is not possible, which should only be the case for one or two species, plants should be sourced from areas that have similar climatic and soil conditions to Waihi.

Plants purchased or grown for planting along the Ohinemuri River should be well hardened to the natural elements before they are planted.

3. Plant numbers

The number of plants to be planted in each zone is easily determined by measuring the square metreage of the area and dividing this figure by 1.5 (metre spacings) twice. For example, if the planting area for the flood zone on the northern bank is 100 metres long and 5 metres wide then 222 plants will need to be planted ([100x5]/1.5/1.5=222)

4. Planting

a Flood zone (zone 2): Plants should be planted right to the waters edge in this zone (that is, the water's edge at normal flow levels), including any slumped areas. The two Carex species should be used most on the water's edge along with some Juncus. The lower half of this zone should be planted with a mixture of the two Carex species, flax, toetoe and Juncus. The upper portion of this zone can be planted with a mixture of karamu, manuka, toetoe, flax and juncus. The species used for the lower part and the upper part of this zone should be gradually merged together so that no apparent distinguishing line results, and this applies equally to where zone 1 and zone 2 plantings adjoin. Carex, flax and Juncus should be planted at the base of eroded banks and tutu planted densely into the face of the banks.

All plants in this zone should be planted firmly to reduce the likelihood of dislodgement during flooding.

b. Upper zone (zone 1): All of the plants in schedule A above are colonising species and can tolerate high light levels and greater competition from weeds. These species should be mixed liberally throughout this zone. Some schedule B and C species could be planted also in the first year in more sheltered areas, although generally it is recommended that these secondary and canopy tree species be planted 1 to 3 years later once the Initial plantings have created some cover and the problem weeds are under control. The schedule B and C plants can either be planted where any initial plants have died or. if plant mortality is particularly low, some colonising species may need to be removed (and perhaps transplanted) to make way for the later species.

The taller growing species should be located where views of the walkway and from the walkway and viewing platforms are not important. Species such as the cabbage tree often have greatest impact when planted together in groves, and this is also the case with tree species such as totara, kauri and kahikatea. The location of tree groves and tree species generally should be indicated on the landscape design plan of the area.

To encourage rapid root development and accelerate bank stabilisation it is recommended that all plants be supplemented with a long-life fertiliser tablet such as Agriform at the time of planting.

5. Post-planting maintenance.

By far the greatest reason for excessive mortality in open site native planting projects is because insufficient post-planting weed control has been done. At least one and quite possibly two release sprayings for weed control need to be undertaken each year following planting and this will need to be repeated every year for two and perhaps as many as four years. Again Roundup should be used with the aim being to keep the one metre diameter planting circle free from grass and weeds. Particular care should be taken to avoid any spray splash or drift onto the native plants as even Roundup can cause the death of natives. Some species, especially toetoe, are very sensitive to Roundup and will die at the merest touch of this chemical.

Any regrowth of blackberry and other invasive weed species should be controlled as soon as they reappear, either by spray or physical removal.

Conclusion

The Ohinemuri River, especially where it passes Waihi township, is a highly degraded and eroding water course that is in urgent need of restoration. The proposed community based H.E.LP. initiative to restore the river system offers the opportunity not only to physically and aesthetically improve the river course but also to establish a repesentative indigenous riparian ecosystem close to the township. There will be considerable educational benefits to be gained from the restoration work and, once restored, should provide a valuable recreational asset for the local community.

It is important, however, that the project has sufficient resources from the start to ensure the best plants are planted as recommended above and that each plant has the best opportunity to survive - Inadequate planning and insufficient resources (funding and labour) will lead to failure.

 

Roger MacGibbon

Indigenous Forest Specialist

Phone/Fax (07) 378 6372, Phone A/Hrs (07) 378 4229 P.O. Box 24, Taupo.

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Waihi District Walkways Inc. PO Box 241 Waihi New Zealand

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