Wednesday, April 28, 2010

Make me a River: Erosion from Fire and Rain

Some of the most spectacular erosion that occurs in Australia is after fire and rain events.


View of Gulaga from Tilba Tilba

On a recent walk up Gulaga (Mt Dromadery) I was lucky to see some amazing river forming processes. Gulaga is situated behind Central Tilba in south-eastern New South Wales, and forms Gulaga National Park. It is sacred to the local Yuin Black Duck people, and is a significant feature in their Dreamtime.


View Larger Map

Gulaga is 787m above sea-level, and consists of 5-separate geologies and several different vegetation communities, including a rainforest at the summit. I was lucky to notice the geology on the walk from some old ANU Geology plaques on many of the rocky outcrops.

Old ANU Geology Self Guided Tour

Erosion from recent fire and flood was evident along the length of the walking track. The Narooma News reported that fire went through the Mountain in August 2009 after a smoldering control burn reignited. The majority of the mountain burnt, apart from the rainforest at the top of the track. Storms in February 2010 caused flooding. The effect of the storm was clear on the unstable landscape of Gulaga. Massive gullies had formed as scars down the side of the mountain. This particular gully was about 12 m wide and at least 150m long and 2-3 m deep. It is the beginning of a river.

Erosion Scar on Gulaga

Australia's rivers, billabongs, streams and swampy meadows are all products of disturbance, erosion and deposition. Recent research by various geomorphologists has shown that Australia's landscape and water features are possibly a result of fire, rain and erosion. This is all very typical of our dry, firey landscape. Disturbance, such as fire, creates an unstable landscape. Loss of vegetation with fire makes soil more prone to erosion from rain. When storms come thought after fires, erosion occurs. The gullies that form could be seen as the beginning of a new river. The soil that is lost will deposit somewhere below, creating a billabong or chain-of-ponds, or perhaps even contibuting to the sand at some of our favourite fishing spots. 

Beach at Mystery Bay: Is this sand from Gulaga?

This cycle is part of natural processes. Usually we think about how erosion is releated to humans modifying the landscape; erosion in agriculture, deforestation or building sites. What we don't often think about is erosion occurring in a place that people have barely ventured. However, erosion does happens naturally! It is amazing to see it first hand in a protected and preserved National Park.

Lynds and I hope to do some research on fire ecology and soils at Gulaga in the next months. 

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