Impact of ground-based timber harvesting on suspended sediment yield in the Sungai Weng Experimental Watersheds, Kedah, Peninsular Malaysia

This paper reports on changes in suspended sediment production in the Sungai Weng Experimental Watersheds, in particular, watersheds W2 and W3, due to ground-based timber harvesting from 1997 to 2002. W3 was logged under normal practice using the crawler tractor-winch-lorry system, whereas W2 was logged using reduced impact logging guides such as a wider 30-m stream buffer strip and 40° slope limit, among others. Logging operations in W3 ran from February 1998 until the end of 2000 and in W2 from June 2000 to March 2003. Suspended sediment concentrations varied within the period of study, increasing dramatically during logging. For instance, the maximum concentration levels sampled in W3 increased from 1980 mg l-1 in 1997 to a peak of 92 500 mg l-1 in 1999. After logging ceased, concentration levels fell to 30 200 mg l-1 in 2001 and to a much lower 19 500 mg l-1 the following year. Peak suspended sediment concentrations were lower in W2, however, suggesting that the extra measures taken to lower the logging impact had some influence on sediment yield in the affected basin. Based on pre-harvesting conditions, data from the Sungai Weng experimental watersheds suggest that conventional logging can increase the suspended sediment yield some 42 times during peak periods, but declines as the watershed recovers.