Many tropical rivers are experiencing large and rapid changes in their water quality. Quantifying those features of the disturbed tropical forest landscape that are the dominant sources of sediment or agro-chemicals requires local hydrological research if commercially-viable forestry pratices are to be modified to maintain water quality. The findings presented illustrate how hydrological research in Malaysia is able to quantify the dominant sources of river sediments in areas distrubed by commercial tropical forestry. This is integrated with a discussion of the ways that so-called 'Reduced Impact Logging' practices in Malaysia can be used to mitigate the worst hydrological impacts.