Two video shootings of lahars in motion were carried out in January 2002 in a river of the Semeru volcano, Indonesia. Through digital video frame analysis, we measured the flow depth, front and fluid surface velocity, and the time changes in the content, size distribution, and transport processes of large boulder particles. Complementary to the in-situ measurements during the flows, we carried out laboratory analysis of samples from a 4-m thick deposit emplaced by the largest debris flow close to the video shooting point. This combined approach allowed us to calculate the sediment deposition rate during the peak flow.