| Lake Burley Griffin Willow Management Plan |
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Areas of Special Consideration
Some sites deserve special mention due to special difficulties or
considerations posed by any willow control program. One such site is the
Molonglo Water-ski area where willows are beneficial in some respects and
detrimental in others.
Water-ski Area
The Molonglo Water-ski area starts from the Dairy Flat Road Bridge over the
Molonglo River and extends for around 5km up the Molonglo River. The main
general public skiing area is approximately 3km long and the final 2km is used
for barefoot and trick skiing by the Australian Water-ski Federation.
The area consists of a very dense infestation of Crack Willow, in association
with other weeds such as Blackberry and poses two major problems for management.
No other species of willow were surveyed along this section and the risk of
establishment of seeding willows is extremely low due to a lack of available
seed beds.
- Willow Debris and Public Safety
Debris from both living and
dead willows poses a threat to water-skiers and onlookers and needs to be
managed on a regular basis. Canberra Urban Parks and Places (CUPP) have an
ongoing responsibility to maintain the water-ski facilities particularly
with regard to public safety, and this includes a program of pruning willows
and managing debris at an ongoing cost.
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| Waterski area bank stabilisation |
- Bank Stability
Willows have been demonstrated to protect banks
against erosion from wave action produced by water-skiing activities. This
has been noted on one site along the Molonglo River where willows were
removed and replaced with (mostly terrestrial) native vegetation. While the
native vegetation grew vigorously and probably went a long way to protecting
the bank areas, rapid undercutting continued to occur and rock armouring of
these unstable banks became necessary. This financially limits any willow
control activities since rock armouring is expensive and produces problems
with backwash for the water-skiers.
Backwash is where waves bounce of the hardened surface and travel back
into the skiing area creating an uneven surface for skiers. CUPP have
undertaken willow control and bank stabilization activities along a few
sections of the water-ski area as a trial with some success. To combat the
backwash problem, coir logs (logs made from coconut fibres) have been
secured in place in front of the rock and aquatic plants have been
established between the log and the rock. In many places it has been
necessary to carefully select aquatic plants that don’t get too tall and
block the view of onlookers. This is being met with mixed results as skiers
are still experiencing problems with backwash. This may improve as aquatic
plants establish and better colonise the area.
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Recommendation 23:
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In the Molonglo water-ski area, rock armouring and revegetation to
combat erosion and wave backwash is expensive, but weighed up against
the cost of ongoing maintenance of willows for public safety, may be
worth further investigation. Further trials should be carried out. |
CUPP are currently undertaking a combination of debris management and willow
removal based on the assessment of specific areas in an attempt to reduce
ongoing management costs, improve biodiversity and make the river area more
accessible for visitors.
Another major benefit that the willows provide is wind shelter, keeping the
water smooth for skiers.
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Recommendation 24:
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Wholesale willow control along the water-ski area is not
economically feasible (even if technically possible) and similarly the ‘do
nothing’ approach is also not feasible from a public safety, maintenance and
access point of view. The approach taken by CUPP to date appears to be a
sensible compromise and will eventually reduce ongoing maintenance costs if
selective removal trials are successful (while continuing to address all the
other management issues). |
Lake Burley Griffin Tributaries
Although the Lake tributaries were not part of the brief for this Management
Plan, they are extremely important as a potential source of reinfestation of
sites where willows have been controlled. It would not take a lot of time to
undertake a quick assessment of these tributaries; especially since riparian
rehabilitation programs (including willow control) have been undertaken in
recent years.
There has been a variety of works undertaken along tributary areas, in the
ACT and NSW, particularly along the Queanbeyan and Molonglo Rivers. Coordination
with NSW catchment managers would be paramount if a serious attempt is to be
made at stemming the spread of problem willow species into the ACT, in
particular Crack Willow.
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Recommendation 25:
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ACT and NSW sections of the Molonglo River and tributaries
need to be assessed with regard to their contribution to willow spread into the
ACT. |
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