Lahar in Glass Creek and Owens River during the Inyo eruption, Mono–Inyo Craters, California

Nested fluvial and lahar terraces are inset in a volcanic surface where Glass Creek has created an embayment in the Sierran range front, on the north rim of Long Valley caldera, California. The geomorphic and stratigraphic relationships among these terraces indicate that they record events at the time of the pre-1472 AD Inyo eruption. The volcanic surface records the deposition of the Inyo pyroclastic materials over pre-existing landforms. Somewhat younger terraces contain volcanic beds at depth, overlain by alluvium. The youngest, lowest terraces comprise lahar deposits overlying alluvium and the pyroclastic debris. Downstream from the embayment, in the canyon of Glass Creek, levees line the walls to a maximum height of 35 m above the present stream level. The canyon opens onto a lahar fan, the surface of which is covered with cobbles and pebbles of Inyo pumice. Further downstream, in the floodplain of the Owens River, an abandoned meander belt is filled with graded debris containing large amounts of Inyo pumice. At the distal end of the floodplain, 30 km from the embayment, coarse pumiceous debris occupies low mounds scattered in the abandoned meander belt. The deposits that comprise the mounds are massive-bedded muds and sandy muds. These features record the excavation of ∼1.9×105 m3 of material from the Glass Creek embayment, and perhaps an additional amount up- and downstream, which was transported in a non-cohesive lahar down the Glass Creek and Owens River drainage to a distance of at least 30 km from the source region. Lahars are a previously uncharacterized volcanic hazard in the Long Valley caldera region. It may be important to consider them in any future assessment of hazards in this restless system.