Two ERS-1 SAR scenes have been acquired over Mt. Pinatubo, before and during the rainy season, respectively on 9 July and 13 August 1993. The last one is coeval with typhoon and subsequent lahar activity. From field observations and consideration of physical properties of lahar surfaces, we decipher keys to detect areas covered by recent and active lahars. The main physical characteristics to consider are surface roughness, soil moisture, and morphology. Color compositions show differences between the two dates. Recently deposited lahars may have high or low roughness at the surface and are wet. Running lahars have a tumultuous rough surface. Recently deposited or running lahars have high backscatter values which differ from low values of more ancient flat smooth-surfaced dry lahar sediments or lahars covered by shallow quiet water. Shallow quiet waters covering more ancient rough lahar surfaces are also detected. Anastomotic streams, fanlike patterns, closure or infilling of lakes, and migration of streams are also morphologic expressions of lahar deposits. Flooded areas during typhoons can be recognized from their location in large coastal plains, while lahars occur along main river beds and nearby.