Willow Control
The most common large tree species found growing along the banks of rivers and streams throughout Southland is Crack Willow (Salix fragilis). Although this species of willow spreads easily from fragments, it is used extensively for river management and erosion control on large rivers and small streams because of its fast growth rate and deep rooting system. The species also provides overhanging cover for fish, assists in maintaining lower water temperatures through shading, and is a valuable source of insects for trout food.

Because the species is prolific and prone to unwanted spread, various works methods are used to ensure control against unwanted spread is achieved and effective soil stabilisation and river management is maintained.

Trimming is carried out to maintain willow growth normally found on small to medium single thread channels, and involves the trimming of overhanging inside growth that may reduce waterway capacity or collapse into the channel. To effectively carry out the work, machinery is sometimes required to work from the channel bed and in the water.

Some level of discolouration to channel flow can be expected from this activity with the degree depending on the amount of silts in the bed material, and time spent by machinery working from the channel.

Removal is normally required on river systems where large willows are likely to, or have collapsed into the channel. The work involves the use of heavy machinery either working from the bank or occasionally the bed and is intermittent, therefore disruption to water clarity is relatively short term.

Cut and layering and Cut and tying of willows is undertaken to strengthen existing willow erosion protection. This work is always carried out from the bank edge therefore effect on water clarity is minimal.

Heavy willow protection involves placement of large willows complete with roots along eroding sections of large river channels. The work does have a short term adverse effect to water clarity.

River Control
Rock protection work is carried out on eroding banks where repair to prevent further major damage is required. The work is intermittent and usually produces a short term isolated effect on water clarity.

Channel diversion is required to the channel within the active river bed. The majority of diversion work is undertaken without the need to work in flowing water, therefore disruption to active river bed and effect on water clarity is minimal and short term.

When diversion takes place in flowing water, a short term adverse effect on water clarity downstream of the activity may be experienced.

These works require resource consent. When such work is proposed, Fish and Game Southland is involved in the process and development of conditions such as timing.

River bed vegetation control is carried out to ensure the open fairway of major river systems are kept clear of woody vegetation that may effect the free passage of flood flows through the system. The majority of work is control of seedling gorse, broom and willow using herbicide applied by helicopter or ground based operation. No herbicide is applied over water.

The work is intermittent; disruption to recreational users of the rivers involved is limited to short term duration of the activity over sections of river. Trout may be temporarily spooked by low over fly of aircraft.

Drainage Work
Drainage maintenance is carried out to provide continuous outfall of farm drainage networks. The work involves the removal of aquatic weed and accumulated silt by excavation, or control of aquatic weed growth through application of herbicide.

The large majority of drainage network maintenance is undertaken on small farm drains that, although are important habitats of native fish, trout, aquatic insects and used by trout for spawning they are low in value to anglers therefore, the high level of discoloured water produced by the excavation of silts is not an issue to recreational users.

Small Work
This category of work relates to isolated single location activities such as installation of culverts and headwall structures, repair to existing structures, and removal of minor gravel deposits that prevent outfall from piped or open drainage systems.

Potential adverse impacts on water clarity and bed disturbance are in the zero to minor range.

[Catchment Home]  -  [Works Programme - detail]  -  [River Events]
[Rivers & Rainfall]  -   [Navigation and Maritime Safety]
[Fly of the Month]  -  [Feedback]  -  [Contact]