LANDSLIDE DAMS: FACILITATORS OR FOILERS OF VALLEY EROSION?

Like all dams on rivers, landslide dams profoundly affect fluvial systems, both directly and indirectly. Mainly studied for their associated hazards such as outburst floods, attention has broadened in recent years to assess the overall affect of landslide dams on river and valley evolution at a variety of temporal and spatial scales. Landslide dams encompass a broad range of sizes, from small blockages ranging up to the 2-2.5 cubic kilometer Usoi landslide dam in Tajikistan, which in 1911 formed the world’s tallest dam (of any type) by blocking the Bartang River valley to a depth of more than 550 m. Most such large blockages are in tectonically active mountainous areas, but landslide dams can influence fluvial systems in quieter tectonic environments, such as the volcanic tablelands of the interior Columbia basin. Ongoing studies in this region are showing that landslide dams are ubiquitous (but more common in specific stratigraphic settings), many but not all produce outburst floods, they can have persistent effects on channel and valley morphology, and they may exert long-term influence on river valley incision. These influences vary among settings and chiefly owe to two competing effects: (1) inhibiting valley erosion by delivering material (commonly coarse) directly to the valley bottom, ponding water and trapping sediment upstream and slowing valley incision downstream by reducing flux of sediment that can act as an erosional tool and by armoring channel beds with coarse material; and (2) accelerating valley erosion by breaching cataclysmically, which can cause significant incision and valley erosion downstream depending on the magnitude and duration of the flood and the erosional thresholds required for valley erosion. Which processes are most important depends on many factors related to geologic and geomorphic setting, and the size and geometry of the landslide and blocked valley. While the hazards of landslide dams at human timescales still motivate most analysis, these indirect effects of essentially instantaneous events are being increasingly recognized as an important cog in how fluvial systems interact with hillslopes to produce landscapes over geologic time.