23 November, 2013

Eaten away

Erosion is a problem common to many rivers and it comes in three main forms: bank erosion, gully erosion and hillslope erosion. Whilst the Barwon is subject to all three forms, studies have shown that by far the biggest impact comes from bank erosion.
Riverbank between Pollocksford and Merrawarp Roads
As the products of erosion enter the river system, they take one of two paths. The lighter matter remains suspended in the water and is carried downstream whilst the heavier material forms what is known as "bedload" which sinks to the streambed to be washed down more slowly. Whilst the suspended material may be deposited on flood plains, ultimately increasing soil fertility, it can cause a reduction in water quality as well as problems downstream, particularly in the shallow lake systems below Geelong.
Bedload can build up on the stream bed further upstream, restricting the flow of water and movement of fish and other river fauna as well as adversely impacting on their habitat.
Prior to European settlement, the river environment was believed to be relatively stable, however from 1836 onwards, clearing of native vegetation, the introduction of stock which impacted soil quality and bank stability and the exploding rabbit population burrowing into banks all added to a rapid increase in erosion along the Barwon. Other contributing factors included the draining of wetlands and marshes for farming purposes and the diversion of water from the Barwon and its tributaries.
The West Barwon Dam
Early efforts to control erosion included the planting of willow trees which it was believed would stabilise the banks. Unfortunately however, they - and any number of other exotic plant species - had the opposite effect, not only contributing to the erosion, but also impacting upon nutrient levels in the water and destroying the habitat of native animals. As their dense roots extend into the riverbank, burrowing animals such as platypus are unable to dig their burrows. As I have blogged previously, by 1949 willows were found to choke the stream, causing flooding, erosion and silting. Today, only a few willows remain here and there along the river.
Another introduced species which can impact riverbank stability is the introduced carp, which not only eats the plants necessary for the survival of native fish, but with its aggressive feeding habits can cause bank erosion. Various initiatives such as "Catch-a-Carp Day" and the draining and dredging of Reedy Lake where a tonne of carp was removed help to control levels of this pervasive pest. If caught while fishing, it is illegal to return carp to the river.
A carp grazing along the riverbank at Marshall
If not used with care, speed boats can also contribute to bank erosion. Speed limits are used to help minimise these effects.
In some cases, natural events also contributed to the problems. When a landslide blocked the channel of the East Barwon River in 1952 causing the creation of Lake Elizabeth, water flow was significantly impacted. The following year when the blockage was partly breached, soil and debris was washed downstream, covering lower-lying farmland. Major flooding in 1995 also caused significant erosion.

Aerial view of the 1995 floods looking south west from the Geelong Advertiser
By the 1940s, efforts were underway to control erosion (whether natural or manmade) and improve water flows and water quality by stabilising the banks and bed of the river. In the 1970s further measures were taken such as the installation of silt traps and revegetation. Following the 1995 floods, intensive measures were required to control and reverse the damage done by the increased water flow.
Today, ongoing measures to minimise erosion and improve water quality are in place and a multitude of Landcare groups across the region work in conjunction with the Corangamite Catchment Management Authority to rehabilitate the Barwon.
Bank and gully erosion can be reasonably effectively controlled in rural areas by restricting the access of stock to rivers, creeks and gullies and by revegetation along riverbanks and areas likely to suffer from erosion. Education of landholders and improved farming methods are also important.
Hillslope erosion is best controlled by encouraging good ground cover improved soil structure which can be aided by measures such as the retention of stubble from cereal crops and the use of raised-bed farming.
Sheep on the river between Pollocksford and Merrawarp Roads
I have noticed in recent times (hence the idea for this post), that bank-stabilisation works are currently being undertaken near Breakwater where rocks are being used to line sections of the bank. Upstream at Queen's Park, some removal of exotic suckering species of trees such as ash has occurred, however I don't know whether this is for stabilising purposes.
Bank stabilising near Barwon Valley Golf Course

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