The role of extreme events in the impacts of selective tropical forestry on erosion during harvesting and recovery phases at Danum Valley, Sabah

Ten years' hydrological investigations at Danum have provided strong eviden ce of the effects of extremes of drought, as in the April 1992 El Nino sout hern oscillation event, and flood, as in January 1996. The 1.5 km(2) undist urbed forest control catchment experienced a complete drying out of the str eam for the whole 1.5 km of defined channel above the gauging station in 19 92, but concentrated surface flow along every declivity from within a few m etres of the catchment divide after the exceptional rains of 19 January 199 6. Under these natural conditions, erosion is episodic. Sediment is dischar ged in pulses caused by storm events, collapse of debris darns and occasion al landslips. Disturbance by logging accentuates this irregular regime. In the first few months following disturbance, a wave of sediment is moved by each storm, but over subsequent years, rare events scour sediment from bare areas, gullies and channel deposits. The spatial distribution of sediment sources changes with time after logging, as bare areas on slopes are revege tated and small gullies are filled with debris. Extreme storm events, as in January 1996, cause logging roads to collapse, with landslides leading to surges of sediment into channels, reactivating the pulsed sediment delivery by every storm that happened immediately after logging. These effects are not dampened out: with increasing catchment scale. Even the 721 km(2) Sunga i Segama has a sediment yield regime dominated by extreme events, the sedim ent yield in that single day on 19 January 1996 exceeding the annual sedime nt load in several previous years. In a large disturbed catchment, such roa d failures and logging-activity-induced mass movements increase the mud and silt in flood waters affecting settlements downstream. Management systems require long-term sediment reduction strategies. This implies careful road design and good water movement regulation and erosion control throughout th e logging process.