The lahars, formed of water, ice, pumice, and other rocks, incorporated clay from eroding soil as they traveled down the volcano's flanks. They ran down the volcano's sides at an average speed of , dislodging rock and destroying vegetation. After descending thousands of meters down the side of the volcano, the lahars followed the six river valleys leading from the volcano, where they grew to almost four times their original volume. In the Gualí River, a lahar reached a maximum width of .