CAN'T GET OVER IT: CAUSES AND CONSEQUENCES OF GEOLOGICAL DISCONTINUITIES IN RIVERS

Fluvial discontinuities are features that interrupt the flux of water, sediment, wood, and other organic and inorganic constituents downstream. The most dramatic of these are geomorphic blockages and dams, which can persist over decades to centuries or longer, and affect a wide range of fluvial processes, both upstream and downstream. Such blockages can result from slope failures along valley walls, including landslides, rockfalls, and earthflows, debris flows and lahars from tributaries, and lava and pyroclastic flows that enter river canyons. The first-order controls on whether or not a blockage forms are: 1) the ratio of influx to efflux – the ratio of the volumetric rate of delivery of material to the rate at which it is removed by the river; and 2) the width to depth ratio of the valley. The most persistent blockages occur where large volumes of material enter narrow canyons. Blockages can be tight or leaky, and be removed slowly or catastrophically. Drawing on examples from rivers around the world, we examine different blockage types, consider factors contributing to persistence or failure, and explore implications of these blockages for long-term evolution of both fluvial profiles and riverine ecosystems.