4. Rockslide and Rock Avalanche Dams in the Southern Alps, New Zealand

Large (>10⁶ m³) rockslides and rock avalanches are common causes of river blockage in the tectonically active Southern Alps of New Zealand. Geomorphic data compiled from 43 examples show that evidence of rockslide dams occurs in comparable number on both sides of the main divide despite differing lithology, rates of uplift, erosion, and sediment yield. These controls however constrain the preservation of few rockslide-dammed lakes in the Southern Alps. In general, well-defined cross-valley dams are between 10 and 400 m high, 1.2 km wide on average, and formed by catastrophic failures. In contrast, many deep-seated low-displacement failures >10⁷ m³ formed ephemeral dams and occlusions, i.e. channel diversions around their toe zones. Rockslide and rock avalanche dams are often major sources of sediment to alpine rivers, though concentrated on a few percent of catchment area. Averaged long-term (>1,000 year) discharge rates from eroding dams can attain 8 × 10⁵ m³/year. Despite the low population density in the Southern Alps, some hazards from rockslide and rock avalanche dams, i.e., outburst floods and prolonged river-bed aggradation from eroding dams remain for downstream communities. The currently high probability of a large earthquake makes synchronous blockage at several locations in the region likely.