Lahars follow the pre-existing topography and descend river valleys. They can show great variation in thickness and are among the most dangerous of volcanic hazards. Renewed magmatic activity is not a prerequisite for the generation of lahars, they can be generated as an aftermath of phreatic eruptions, or from heavy rains accompanied by seismic disturbances. The destructive potential is immense, as exemplified by the Nevados del Ruiz disaster.
Mudflow hazards are documented at Volcán Fuego, where lahars on many south-flowing rivers were generated from glowing avalanched deposits of the 1974 eruption when rains occurred. Downslope bridges are destroyed by rains after every eruption of Fuego. Especially extensive laharic deposits from Fuego on the coastal plain are also described.
Sapper described cold mudflows generated by the removal of unconsolidated ash material from the 1902 Santa María eruption. Kuenzi et al. describe even more dramatic disruptions affecting the Samalá river system lasting many years after the same eruption. The 1929 activity generated extensive pyroclastic flows along the Río Tambor, and smaller flows have occurred periodically since. The secondary mudflows related to pyroclastic flows and domes extrusion at Santiaguito have consistently affected the Samalá River and its tributaries and have been a particularly difficult problem during the low level activity of Santiaguito since 1975.
Mudflows and flooding which results from the attendant disturbance of drainages constitute the hazard of greatest likelihood for Tacaná, because with the dramatic graviational gradient, they could be extensive even without a magmatic eruption. If magmatic eruption does occur, lahars are a near certainty and should be expected to occur sporadically for months, or even years, after the eruption ends. Removal of unconsolidated ash from river valleys will be fast if an eruption takes place during the rainy season, thus increasing the immediate hazard of flooding and lahar generation. Should the eruption take place during the dry season, removal of ash material by rainfall will be delayed until the rainy season, a time when the volcanic hazard conciousness of the local population may have subsided. Even though many Guatemalans are familiar with volcanic hazards, the absences of historic activity at Tacaná means that the degree of hazard familiarity of the local population is uncertain.
Mudflows and flooding can be expected downstream from any flank affected by eruptive activity. They can also be expected from all flanks in the event of a summit eruption. In the event of activity focused on the NW flank of Tacaná. as was suggested by 1986-1987 events, relatively little impact will be felt on the Guatemalan side, but extensive mudflows and flooding could occur downstream from Río Agua Caliente and Río Coatán. Population centers potentially threated by this hazard are those along the drainages of Río Las Majadas and Río Tocanaque in Guatemala, and Río Agua Caliente, Río Coatán, Río Zapote, Río Maxaum, and Río Suchiate in Mexico.