Streamwater quality following logging and wildfire in a dry sclerophyll forest in southeastern Australia

Abstract

In an 8-year study in two large adjacent forested river basins located in southeastern Australia, streamwater cationic composition was found to be greatly influenced by the mineralogy of the major lithology in each basin. While sodium was the dominant streamwater cation in each basin, it was relatively much higher in the Wallagaraugh basin in which the major bedrock contained considerably greater proportions of potash feldspar than in the Towamba basin. The chemistry of bulk precipitation, while similar in each basin, may have influenced the ultimate ionic composition of streamwater to a greater degree in the Towamba basin than in the Wallagaraugh basin. Streamwater cationic concentrations in small sub-catchments subjected to logging and/or wildfire initially decreased relative to concentrations in an untreated control catchment in response to relative discharge increases, but became relatively greater than pre-wildfire 3–4 years later. Potassium concentrations in streamwater increased relative to the control in the year of the fire. Changes in the propprtions of cations in streamwater in the small catchments during the study suggested that hydrologic changes resulting from a protracted drought, and from logging and/ or wildfire, may have influenced ionic releases and ionic transport processes in these catchments. While these changes were readily detected in small catchments, they seem to have been largely buffered out in the major rivers. Streamwater turbidity levels were increased in the year of logging by combinations of clearfall logging and wildfire in the small catchment study. Wildfire alone did not increase sampled turbidity levels. All burnt catchments, and especially those logged, experienced a reduction in sampled stream turbidity levels during the post-treatment period such that within 5 years the levels were significantly lower than those in unburnt, unlogged controls. These reductions are attributed to better catchment protection afforded by the dense revegetation of burnt areas which occurred during this period. Turbidity levels in the logged research catchments were in general similar to those in major streams in the Wallagaraugh Valley whose catchments had also been subject to logging during the study period. Streams sampled in the nearby Towamba Valley, which experienced less logging in the period, had lower turbidity values throughout.