Several important river systems stem from Mount Pinatubo, the major rivers being the Abacan, Tarlac, Pasig-Potrero, Sta. Lucia, Bucao, Santo Tomas, Maloma, Tanguay, Ashley and Kileng rivers. Before the eruption, these river systems were important ecosystems, but the eruption filled many valleys with deep pyroclastic deposits. Since 1991, the rivers have been clogged with sediment, and the valleys have seen frequent lahars which continued for years after the eruption. Studies show that the river systems will take decades to recover from the June 1991 eruption.