THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES Publication 1061 October 2006 1. INTRODUCTION South-eastern Australia experienced its worst drought in 100 years during 2002–03. As a consequence of the extremely dry conditions that prevailed, over one million hectares in north-east Victoria was burnt during the largest fires since 1939. A total of six Victorian river catchments were affected (the upper Murray, Kiewa, Ovens, Mitchell, Tambo and Snowy), with most of the burnt land located in State and National parks. Crystal Brook one month after the fires In addition to the effects on livestock and property, the fires also caused significant ecological impacts. While devastation to the land is a highly visible consequence of fire, streams are also affected, because surrounding catchment condition is critical to stream health. Of particular concern are the short and long-term impacts of the fires on water quality and stream life, which to date have not been extensively examined in Australia. Twelve months after the fires EPA Victoria was funded by the State Bushfire Recovery Taskforce to undertake a series of studies following the fires. This report is a regional summary of the changes in stream health that occurred at 32 sites in the three years following the 2003 bushfires. Three years after the fires THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES 2. THE IMPACT OF FIRE ON STREAM rivers and streams). A comparison between pre and HEALTH post-fire conditions provides a sound basis for our assessment of bushfire impact. Although the immediate impact of a fire on a stream A suite of biological indicators was calculated from is relatively small, there are several key ways in the RBA and then aggregated to provide a combined which stream ecosystems can be affected in both stream health score for each site in each year. This the short and long term, including: score is similar to that used in the Index of Stream • sedimentation – loss of vegetation coupled Condition (a statewide benchmarking of stream with changes in soil structure can lead to a condition developed by DSE) but differs in using four build-up of loose sediment and ash that can rather than just two biological indicators. be rapidly washed into streams during heavy rains (a sediment ‘slug’). These sediments scour and smother the in-stream habitat, reduce dissolved oxygen and light penetration in the water column, and clog the gills of fish and other aquatic biota. • algal blooms – greater amounts of sediment entering the waterways also increase the concentrations of nutrients within the system. Higher levels of nitrogen and phosphorus, coupled with reduced stream shading, can lead to excessive plant and algal growth. 3. REGIONAL ASSESSMENT OF STREAM HEALTH IN THE THREE YEARS SINCE THE FIRES Macroinvertebrate sampling in the alpine region, Spring 2004 The health of streams within the fire-affected region If a site showed a significant reduction (greater than was assessed using aquatic macroinvertebrates 20 per cent) in its stream health score in any year (such as insects, snails and worms) as indicators of after the fire then it was assessed as impacted. Sites environmental condition. that recovered to their pre-fire condition (or within The macroinvertebrates were collected in autumn 10 per cent) by the second or third year of the study and spring in a year preceding the fires, and in each were defined as temporarily impacted, while any of the three following years, using EPA’s rapid sites that had still not recovered by the end of the biological assessment (RBA) method (see EPA third year were considered as showing a continuing publication 604, Rapid bioassessment protocol for impact. EPA Victoria 2 THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES A year-by-year summary indicates that impacts were most prevalent in the first year after the fires, 80 affecting more than 30 per cent of sites, while 60 stream health improved significantly in the second 40 and third years of monitoring. 20 However the effect of the fire on individual sites was Percentage of Sites 0 highly variable. Some sites showed no decline in No Change Temporary Continuing stream health in the first year but then a reduction in Impact Impact the second or, in the case of the upper Tambo River, third year of the study. Percentage of sites impacted by the fires Nearly half of the 32 sites experienced some form of While the condition of many streams did show a impact after the 2003 bushfires. Impacted streams decline, there was no direct correlation found occurred in all catchments (except the Mitchell) and between fire severity and changes in stream health. included a broad range of stream types, from small Other factors such as subsequent rainfall events, alpine creeks, such as Crystal Brook at Mt Buffalo, to fire patchiness, and the presence of streams large, broad, lowland streams like the Ovens River at unaffected by fire that could be a source for Whoroully South. recolonisation, appeared to be more important in determining fire impacts. This was apparent after Only one site, located on the upper reaches of the the first year of monitoring (see the EPA publication, Tambo River, was still found to be impacted three River health – A snapshot of the effects of the 2003 years after the fires, although three sites further bushfires) and a further two years of sampling has downstream on the Tambo had all recovered by the supported this conclusion. end of the third year (refer to the following maps). 40 30 20 10 Percentage of Sites 0 2003 2004 2005 Year Percentage of sites showing a reduction in stream health when compared to pre-fire levels, in each of the three years after the 2003 fires Information Bulletin 3 THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES Murray River (! Ovens Wodonga River / Kiewa River Ovens River (! Kiewa (! Ovens (! Buffalo River !( (! (! (! Bright (! (! (! (! King River (!(! (! (! Dargo Change in Stream Health River (! (! Sample Sites (! No Change (! Temporary Impact Fire Affected Area (! Mitchell Mitchell River Bairnsdale (! 05 10203040 Kms Map of Ovens, Mitchell and Kiewa Showing Changes in Stream Health EPA Victoria 4 THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES Murray Change in Stream Health River Sample sites (! Continuing Impact Tallangatta (! (! Corryong (! No Change (! Temporary Impact Fire Affected Area (! Mitta Mitta River (! (! Upper Murray Gibbo River Big (! River Mitta Mitta River Limestone Middle Creek Suggan Buggan River (! (! River (! (! Tambo (! River Victoria River Omeo (! (! (! (! Tambo River(! Buchan River Tambo (! Snowy (! Orbost - (! (! Lakes Entrance 05 10203040 Kms Map of Tambo, Snowy and Upper Murray Catchments showing changes in stream Health Information Bulletin 5 THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES 4. STREAMS AFFECTED BY to escape from the water (see the EPA publication, SEDIMENT SLUGS The impacts of bushfires following a flash flood event in the catchment of the Ovens River). Many streams were affected by ‘sediment slugs’ in the three years following the fires, including the Tambo, Ovens, Buchan, Suggan Buggan and Mitta Mitta rivers. These sediment slugs were primarily due to high rainfall events washing exposed topsoils into streams. 80 60 A sediment slug in the Ovens River at Whorouly 40 20 0 Percentage of Sites No Change Temporary Continuing Impact Impact Effect of sediment slugs on stream health following the 2003 fires Almost 80 per cent of sites on streams known to have been affected by sediment slugs declined in condition at some point after the fires, nearly double The aftermath of a severe sediment slug that moved the percentage impacted in the fire region as a through Livingstone Creek, near Omeo, in April 2003 whole. Initially these sediment slugs affect stream health 5. EPA’S FINDINGS by drastically reducing dissolved oxygen levels and The 2003 bushfires had a negative impact on many through scouring of the streambed. As the slug streams in eastern Victoria. The fires occurred passes through, large amounts of sediment settles towards the end of a long drought, the worst in 100 out, smothering the bottom of the streams, an years, which may also have contributed to the important habitat for both macroinvertebrates and decline in stream health in the fire-affected region. fish. Other streams assessed by EPA in unburnt areas For example, oxygen levels within the Ovens River (such as the Latrobe, Tambo and Upper Yarra during a sediment and ash slug in 2003 fell to zero catchments) have remained in good condition for almost a day, killing fish and prompting crayfish throughout the study period, despite the drought, EPA Victoria 6 THE HEALTH OF STREAMS IN NORTH-EASTERN VICTORIA IN THE THREE YEARS FOLLOWING THE 2003 BUSHFIRES suggesting that the bushfires are the primary cause the slug and subsequent flushing flows may all play of the changes observed in stream condition a part in this relationship. throughout the fire-affected region. Unlike sedimentation caused by poor land Approximately half of the streams assessed did not management, fire-derived sediment impacts appear decline in condition as a result of the fires and all to be relatively short term. Most of the fine but one of the sites that were impacted had sediments have now washed downstream and been recovered by the third year after the fires. Given that dispersed and sites have recovered physically to fire and drought have been shaping the Australian their pre-fire condition. The Tambo River, which has landscape for millions of years, it is not surprising not fully recovered, will continue to be monitored by that aquatic ecosystems have evolved to cope with EPA.