Last week Dom Blackham and I visited the upper Yarra catchment, which was severely burnt during the devastating fires of February 2009. We were really pleased with the amount of native regeneration we saw in the catchment, especially in the riparian zones. In the years following bushfires there is a heightened risk of erosion as the stabilising vegetation is often lost. In the upper Yarra catchment we were amazed at the amount of gravels and cobbles that have been eroded from hill slopes and gullies. We saw the remnants of huge debris flows, where tonnes of unconsolidated cobbles, boulders and logs have been transported down gullies, eroding more material as they go. A small tributary in the Steels Creek catchment had been completely infilled by coarse sediment. It now looks more like a gravel road than a waterway.
A tributary in the upper Steels Creek catchment which has completely in filled with gravel now resembling a gravel road!
Native regeneration in the riparian zone of Steele Creek
The inflows of coarse sediment appear to have had a dramatic impact on the geomorphic form of streams in the burned areas. The photos below show a creek in the upper Arthurs Creek catchment at the point where tonnes of gravel and cobbles have been delivered from an eroded gully. Upstream of the eroded gully the channel appeared homogeneous with uniform bed form and hydraulic habitat (Photo). Downstream of the sediment inflow the channel form and flow was much more diverse, with pools and riffles forming in the deposited coarse sediment. We also noticed a large volume of large wood that has been delivered to the creeks and stream.
Arthurs Creek just upstream of where the gully erosion has entered the waterway
Arthurs Creek just downstream of where the gully erosion has entered the waterway. Check out the difference in bed material!
Bushfires present some issues for waterway managers in their immediate aftermath, mainly due to the increased occurrence of erosion and flooding. However seeing these systems more than a year after the February 2009 fires suggested that the post-fire effects may actually increase geomorphic and habitat diversity. We left thinking that bushfires might not be all bad news for the health of our waterways.