The effect of steep slope logging on fine sediment infiltration into the beds of ephemeral and perennial streams of the Dazzler Range, Tasmania, Australia

Gravel-filled traps were buried in the beds of streams draining steep logged and unlogged catchments of the Dazzler Range in northern Tasmania, Australia, and removed after storm events, to assess infiltration of fine (less than 1 mm) material into the bed. All stream catchments were geomorphically similar, over similar altitude ranges and had moderately erodible sandy-clay soils on 25-35[o] slopes. Study catchments were selected to control for aspect, logging treatment and coupe age. Fine sediment infiltration into the stream bed was assessed for 15 tributary ephemeral streams in logged areas and 11 streams in unlogged areas. The logged catchments had been clearfelled in three time periods-1990-1991, 1988-1989 and 1986-1987-all by skyline cable logging. Trap yield was also assessed in riffles of the perennial valley floor streams upstream and downstream of the junction of six logged and six unlogged tributaries and upstream and downstream of four old but actively used road crossings. Trap yield was significantly higher in logged than unlogged ephemeral streams for size fractions ranging from less than 125 to 500 um, by factors ranging from two to three, but not for sediment between 0.5 and 1.0 mm. Trap yield of organic sediment of less than 125 um declined with time after logging and burning, whereas inorganic sediment yield showed no clear trend with couple age. Trap yield of streams logged in 1986-1987 was not significantly higher than for control streams, whereas inorganic sediment and 0.5-1.0 mm organic sediment yields were highest for recently burnt coupes. A significantly greater number of increases in trap yield occurred between riffle pairs of valley floor streams adjacent to junctions of logged tributaries, when compared with control riffle pairs. Logged tributary junctions were associated with an increase in the organic content of sediment. Road crossings were associated with large increases in infiltration in adjacent riffle pairs, 30-50 years after construction. Current forest practices do not protect ephemeral headwater streams from enhanced sediment inputs, the long-term significance of which is unknown. Recovery of sediment fluxes in these streams to background levels appears to take 5 years or longer.