WATER-QUALITY IN MOUNTAIN ASH FORESTS - SEPARATING THE IMPACTS OF ROADS FROM THOSE OF LOGGING OPERATIONS

The purpose of the two catchment studies reported here was to allow th e effects on water quality of road use and maintenance to be separated from the effects of a logging operation. In the first project, known as the Myrtle experiment, two small catchments in an old-growth mounta in ash (Eucalyptus regnans) forest were chosen for a paired catchment study of the effects on physical and chemical water quality (baseflow and stormflow) of logging under a strict code of practice and with no roads crossing runoff producing areas. In the second project, known as the Road 11 experiment, the effect on sediment production from unseal ed forest roads of vehicle use and level of road maintenance was asses sed. The Myrtle experiment showed that the harvesting and regeneration operation did not have a major impact on the stream physical or chemi cal water quality. Increases were detected in turbidity, iron and susp ended solids at baseflows, but these were small in absolute terms and of similar magnitude to the measurement error. The stormflow data reve aled no significant influence of the logging operation. The suspension of logging during wet weather, the protection of the runoff producing areas with buffer strips and the management of runoff from roads, sni g tracks and log landing areas eliminated intrusion of contaminated ru noff into the streams, thereby avoiding the adverse effects of logging . The Road 11 study determined that annual sediment production from fo rest roads was in the range of 50-90 t of sediment per hectare of road surface per year, with approximately two-thirds being suspended sedim ent and one-third coarse material. The use of gravel reduced sediment production, provided a sufficient depth of material was used. Increasi ng the level of road maintenance with increasing traffic load controll ed sediment production rates, but when maintenance was not increased, sediment production increased by approximately 40%. The results indica te that by identifying the areas that produce runoff it is possible to prevent contaminated runoff reaching the streams. Roads, on the other hand, produce large quantities of sediment, even when well maintained , so careful consideration of their placement and management is paramo unt.