Complex sentence: Pinatubo on it's major eruption on June 15,1991, filling the River with over 10 feet of Lahar, it's main tributary is the O'donnel river in Sta.
Split into simpler sentences: Pinatubo on its major eruption on June 15,1991, filling the river with over 10 feet of lahar. Its main tributary is the O'donnel river in Sta.