Impact of the 1983 wildfires on river water quality in East Gippsland, Victoria

Eleven stream stations within the basins of the Bemm, Cann, Thurra, Wingan and Genoa Rivers were sampled during a 3-month interval following a prolonged drought and intense and extensive forest fires. Emphasis was placed on flows resulting from three major storms that occurred during this period. Water-quality impacts of the fires were intermingled with those of the preceding drought, and flow- related comparisons with pre-drought data showed appreciable increases in colour, turbidity, suspended solids, potassium and nitrogen levels in the Bemm River, which was only marginally affected by the fires. In the Cann and Genoa Rivers, with much larger proportions of catchment burnt, electrical conductivity and phosphorus concentrations also rose substantially. Marked depletion of dissolved oxygen (to 6 mg I-1) was unique to streams with burnt catchments, but resulted from stagnant conditions at the end of the drought as well as from changes occurring at the time of the first post-fire storm. The fires had little obvious effect on temperature and pH regimes. Peak turbidities and concentrations of suspended solids and phosphorus were much greater in the Cann and Genoa river systems than elsewhere. Maximum values for these indicators were 130 NTU, 2300 mg I-1 and over 0.8 mg I-1, respectively. In the Thurra and Wingan basins, which were also burnt, stream suspended-solids levels were lower (200 mg I-1), but solutes sometimes reached very high maxima (indicated by peak electrical conductivities of up to 110 mS m-1). Variations in catchment topography and soils and the relative importance of surface and subsurface flow probably account for these differences. The first post-fire storm produced the highest measured levels of many indicators in most streams, although the greatest flows were associated with the third storm. Nitrite and ammonia were notable exceptions to this generalization. Estimates of catchment exports indicated high sediment yields and moderate to high phosphorus yields from the Cann and Genoa catchments, by comparison with other Australian data.