Background

Implications for Aquatic Habitat and Environmental Flows

Flow regimes in streams are changed as a result of fire, as described earlier. In the short term, peak flows during floods and dry-weather flows increase. In the long term, regrowth of some forest types may cause much lower streamflows. As well, large quantities of coarse sediments may be moved into streams, filling waterholes and altering the streambed character. This happened in many of the small tributaries of the Yarra River after the 1939 fires. Conversely, rapid nickpoint erosion in streams and bank collapse can convert a meandering “chain of ponds” watercourse into a deeply incised gully. Ecologically valuable boggy areas and wetlands may be destroyed through these processes. Local geomorphology and altered water balance determine whether these events occur.

In any case, such changes will affect stream biota. It will be necessary to identify streams where the existing aquatic habitat should be preserved, and undertake the preventative measures needed to preserve the character of those watercourses. These could include short-term remedies, using constructed barriers to detain runoff, diversion banks, silt curtains and the like. Emergency trails constructed during the fires will need special attention and rehabilitation to prevent erosion from them and “put them to sleep”.

After fire, previously-dry watercourses will commence to flow for a few years before they revert to their previous state. Landowners should be aware that this is a temporary change, and their use of the newly-available water should be regarded as a short-term bonus. Additional permanent pumping licences, either from streams or groundwater, should not be allocated.

In the longer term, fire in water-supply catchments is likely to reduce the security of supply for current water allocations. In such cases, there will be more pressure to reduce environmental flow releases, and less chance of increasing the current allocation to the environment. In rivers already subject to dramatic hydrological alteration (e.g., where dams have been constructed), such effects could have serious environmental consequences. These will be more severe on streams where dams that rarely overflow have been constructed.

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