Practices which minimise damage to watercourses
Arable Land and Lowland Pasture
Because these kinds of farming are concentrated on river floodplains and terraces, they coincide with watercourses that cut through alluvial soils. On many lowlands, stream and river banks are also mantled with loess soil or volcanic ash. All three materials are prone to scour and collapse during floods.
Riparian retirement fencing (Figure 12)
| Riparien fencing - keeps stock away from banks and watercourses |
Riparian retirement is carried out where banks are not needed for grazing or watering of stock. The practice entails fencing off banks from cultivation or grazing. Native vegetation if still present is left in place. If not, trees and shrubs may be planted. More usually, banks are simply allowed to revert into rank grass and weed. Impacts on bank erosion and water quality are as 21:
- Complete riparian retirement, along all the permanent watercourses in a catchment, reduces bank erosion and sediment loads by 50% to 90% or more.
- Solutes (dissolved chemical pollutants) in watercourses, mainly N and P, are reduced by similar percentages. The higher reductions are usually obtained where retired strips include swampy areas marginal to channels.
- Organic loading of water supply (fouling by dung, urine and associated micro-organisms) is avoided.
The main factors in achieving these impacts are exclusion of stock from watercourses (which reduces bank trampling and water fouling); and presence of dense bank cover (which traps much of the residual sediment entering in runoff from adjacent pasture). A third factor is that exclusion of stock from watercourses enables recovery of aquatic vegetation and associated micro-organisms. Both fix a proportion of the N and P which enters by groundwater seepage through banks, and possibly even a proportion of the N and P in water flowing from upstream.
In some situations, riparian retirement can cause water quality to deteriorate, for example, where bank undergrowth dies back beneath a tree canopy that is too dense, or where swamp and channel vegetation are excessively shaded. This is unusual under native or deciduous tree cover but has been measured under conifers.
Figure 12: River bank protected from erosion by rank growth inside riparian retirement fence (lower right), compared with eroding bank beyond end of fence (upper left). Waikato.
A disadvantage of riparian retirement is infestation of ungrazed banks by weeds and pests, which may spread into adjacent paddocks. Others are the high cost of fencing, and of providing reticulated stock water supply to paddocks. On a large farm, riparian retirement of all the streambanks at once is quite impractical, but a staged programme over several years is usually feasible.
There can also be on-farm benefits if riparian retirement fences are carefully sited, for instance avoidance of stock losses through drowning or bogging, and easier mustering where stock do not have to be moved along rough banks or through channels. Interestingly, stock on some farms (particularly dairy and beef) have been observed to stay away from watercourses most of the time if they have water supply troughs, bringing at least a partial improvement in water quality without the expense of fencing.
If grazing or water access for stock are needed, collapse and scour can be controlled by planting non-suckering, deep-rooting tree species at spacings of 6 to 12 metres along unfenced streambanks (closer on weak points). The most appropriate species currently available are rust-resistant hybrids of European and Asiatic poplars. Alders, gums and wattles are alternatives. Shrub (osier) willows are less desirable, as their bushy growth form may suppress pasture growth and block access. Large tree willows, and silver poplars, are to be avoided as they split, fall into channels, sucker and spread downstream. Tree planting has the following impacts22:
Figure 13: Streambank protected against erosion by thick mat of tree roots, Manawatu. Photo: MWRC
- Reduction of bank erosion by 50 to 80%, relative to what can be expected on unplanted banks. This controls but does not eliminate siltation (because runoff still washes a little soil from grazed pasture on the banks).
- No effect on solutes (dissolved chemical pollutants are still washed from adjacent pasture),
- No effect on fouling of water supply (stock still enter the channel).
Where native vegetation still exists on streambanks (whether trees, scrub or tussock), leaving it there can be just as effective a protection as planting exotic species. Replanting native species on cleared banks is also an option. It was successfully used by former government departments such as the Ministry of Works and Department of Lands, to rehabilitate streambanks after construction work or land development. A few private farmers are also known to have undertaken bank planting with native species.
Controlled grazing (Figure 14)
| Controlled grazing - suppresses weeds on banks, and still maintains water quality for most of the time |
If grazing or water access are not needed through the year, but fencing costs or weed growth are a problem, a simple solution is to erect electric fences, and graze heavily once or twice a year to suppress weed growth. On straight reaches and inside bends, rank grass can be just as effective a protection against bank scour as tree roots. On outside bends (weak points), some reinforcement with tree roots or rock work will be needed. Quite a number of farmers already graze streambanks in a controlled fashion, but scientific measurements of its effects are not known to have been made on their properties. The impacts are likely to be:
- just as much reduction in bank scour as achieved by riparian retirement fencing or continuous tree planting;
- confinement of sedimentation, chemical pollution, and fouling of watercourses to a small percentage of the year (perhaps a small price to pay for adequate weed control along banks);
- lower fencing costs (light posts, two wires, and no battens);
- lower tree planting costs (planting can be confined to weak points).
A modification sometimes practised on mixed-stock farms is to use a single electric wire, sufficient to keep out cattle but allow sheep (which do less damage to banks) through for weed control. This substantially reduces sedimentation, but has little effect on chemical pollution in runoff from pasture, or on fouling of water by dung and urine.
Pastoral Hill Country
Different techniques are needed to control bank erosion on hill country, because the character of watercourses is different. They tend to have steep gradients, and so are liable to gully. Bed and bank materials are more likely to be rock (albeit weathered and easily scoured) than alluvium. In hill country watercourses, cattle cause more damage togully sides than sheep. Riparian retirement or controlled grazing are feasible only along main streams flanked by valley-bottom terraces. On slopes, watercourses are so numerous that to attempt permanent or temporary fencing would not just be prohibitively expensive, it would be completely disruptive to paddock management. As alternatives, the following management practices can at least minimise sediment input from hillside tributaries into valley-bottom channels.
Figure 14:
River bank protected against erosion by dense grass and weed inside electric fence, with removable wire to allow occasional grazing. Waikato.
Spaced plantings (Figure 1.5)
| Spaced planting - with trees close enough for roots to interlock along channel |
Deep-rooting trees can be space planted up watercourses where there is some risk of gullying, and also along the lines of sub-surface tunnels (under-runners). The technique is referred to as pair planting where trees are planted on opposite banks or alternately on each bank. On many small ephemeral watercourses, which only flow after heavy rain, a single line of trees within the channel is sufficient. Poplars and willows are the main species used, but in dry districts where they have difficulty establishing, eucalypts and acacias are increasingly used instead. Trees need to be spaced no more than 12 metres apart (4 to 6 metres is standard). Spaced planting on hillside watercourses23
- reduces the length of watercourse damaged during floods by anything from 30 to 80%, (depending on nature of bank materials, and the proportion of watercourses where trees are adequately spaced);
- produces a 10-20% reduction in extent of mass movement on adjacent slopes (even though these remain unplanted);
- has minimal impact on pasture production (see earlier discussion under heading "Practices which reinforce soil").
Figure 15: Incipient gullies stabilised by space-planted poplars. Stock can still graze underneath. East Coast. Photo: D Miller
Debris dams (Figure 16)
| Debris dams - closely spaced, low structures, not widely spaced high ones |
Dams are used on watercourses where gullying has already occurred, to stabilise sediment on eroding gully floors while trees establish. They are constructed out of timber and wire netting. Early designs used to be expensive, but in recent years, simple half-round and netting structures (and even single planks pegged across gully floors at close spacings) have been found cheaper and equally effective. An equivalent technique for stabilising sub-surface tunnel gullies (under-runners - see page 9) is to fill collapse holes with slash or haybales before planting trees. Bulldozing, followed by surface cultivation, has occasionally been used on areas of particularly dense tunnels, but is an expensive remedy. Debris dam (24), in combination with spaced or close planting of trees:
Figure 16: Open gully initially stabilised by debris dams, to enable spaced planting of willows. East Coast. Photo: G Shepherd
- reduce the amount of fresh gullying by 30 to 80%. depending on nature of bank materials, and adequacy of dam and tree installation;
- can have a beneficial effect on pasture growth and stock carrying capacity, in the sense that checking gully incision helps prevent slips or earthflows in pasture on either bank.
Close planting (Figure 17)
| Close planting - a continuous, dense root network prevents deep gullies from gnawing upslope |
Particularly severe open gullies on mudstone or clay country can be fenced off and close-planted with trees. They are lost to grazing, but this at least stops the gully from eating headward into good grazing land. Sometimes trees can be harvested from the gully margins, so it is a good idea to plant these with commercial timber species such as pine or blackwood. However it is not such a good idea to harvest timber from the unstable gully walls or floor, in case erosion starts again. They are best left to revert to native cover or if severely eroding, planted with willows, poplars, gums or wattles to speed up stabilisation. Where close planting has been implemented, the following impacts occur (25):
- Livestock production foregone is proportionately much less than gully area. (Severe gullies generally have stock-carrying capacities of 1 stock unit a hectare or less, due to poor pasture growth and difficult access);
- Headward expansion of gullies into good farmland is prevented. For instance, in one Wairarapa catchment where 284 gullies were retired and/or close planted (4% of farm area), another 23% of farm area was estimated as saved from gullying or gully-triggered mass movement over the following 25 years. On one large East Coast property, four untreated gullies expanded from 33 to 49 hectares between 1956 and 1982. Another four gullies occupying 26 hectares were retired and close planted at various dates. Gully expansion not only ceased; active gullying in the retirement blocks contracted to 11 hectares.
- Sediment input to downstream watercourses is reduced. By way of example, close-planted gullies reduced sediment sources by 35% catchment-wide during the 1991 Tinui storm. In gullies where conifers were planted right to the banks, reductions averaged 22%. Reductions averaging 78% were achieved in close-planted gullies where native cover, or willows and poplars, were retained along banks in the middle of pine blocks.
Figure 17:
Open gully stabilised by close-planting of pines on slopes either side, and re-establishment of native undergrowth on banks. Wairarapa.
Tussock High Country
Control of streambank damage on high country runs requires a mix of management practices. On the broad valley bottoms with their fill of rolling glacial till or terraced outwash gravel, similar practices to those recommended for lowland pasture can be used, but they are subject to constraints?
Riparian retirement
This is technically effective in maintaining dense bank cover (tussock and scrub) with associated reductions in solute loading and water fouling. Measured reductions in bank erosion range from less than 10% on some reaches to more than 80% on others. In both situations, the effect on the amount of sediment carried by streams is probably minor, given the high sediment load already in the channels from their mountain headwaters.
Tree planting
Poplars and willows can be established at weak points on banks. However the size and ferocity of high country rivers in flood, particularly the tendency of their braided channels to change course, limits the effectiveness of tree planting as a means of bank protection.
Temporary fencing with occasional grazing
The same comments apply as for riparian retirement.
On high country slopes where steep, high-energy torrents cut through loose rock debris, similar practices to those recommended for pastoral hill country can be attempted, but the constraints on streambank management are even more drastic27.
Spaced tree planting
Cold-climate willow and poplar species can be established on streambanks crossing lower slopes, but few are likely to survive at high altitude (those that do, are sufficiently hardy to constitute potential weeds). Grazing by pests, such as possums and hares, also greatly reduces tree survival. Native species, notably beech, will establish up slopes as far as the treeline, but are unlikely to stabilise the banks of torrents against flash floods or debris flows.
Close planting
Similar comments apply as for spaced planting. A large number of coniferous species can be successfully established around gullies on lower slopes, and a few can even grow at high altitude (though these often spread as wildlings). However they, and even native species naturally occurring on torrent banks, are unlikely to survive undermining by floods and debris flows.
Debris dams
Timber and netting structures will be demolished. The only structures capable of stabilising debris in mountain torrents are concrete check dams, of the type used in the European Alps and Japanese mountains. The cost of construction is such that their use could only be justified where they protect communications routes or buildings.
The sheer size of high country runs renders any of these techniques prohibitively expensive over an entire property. While examples are sometimes seen, usually they are on short reaches, and the motive for erosion control has been maintenance of road access or improvement of wildlife habitat, rather than protection of farmland.
Contact for Enquiries
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