Sediment response to catchment disturbances

Abstract

1 IntroductionThe surface of the planet undergoes a constant battle forbalance between uplift and erosion. Plate tectonics causeland to be created and mountains to be born, whileweathering and erosion act to lower the land surface. Riverstransport the products of weathering and erosion to storagepoints, or sinks; the ultimate sink being the deep oceans. Fora given landscape, an equilibrium is reached whereby therates of sediment distribution between sources and sinks arefairly constant. In the case of a river channel, the amount ofwater and sediment moved through a landscape is balancedby the energy available to do this, such that a change ineither of the two generates adjustments in stream power. Achange in environmental conditions often results in aresponse, the magnitude of which is a function of the typeand intensity of “disturbance” to the system. In the case ofthe river channel, this might manifest itself in the form ofchannel aggradation or degradation, whereby sediments areeither deposited or eroded in response to changes in availableenergy or sediment supply.Changestoalandscapearebothnaturalandanthropogenic.In terms of landscapes, we tend to think of a “disturbance”eventasonethatisfairlyextremeandproducesameasureableresponse in the rate or type of processes occurring in thelandscape. Examples of natural disturbances include earth-quakes, volcanic activities and wildfires. Examples ofanthropogenic disturbances include deforestation, intensiveagriculture, urbanization, dam construction and gravel min-ing. Typically, in natural systems, the types of geomorphicresponses to these events include increased rates of sedimentmobilization (i.e., soil erosion, mass movements), transport(i.e., delivery to and within river channel systems) anddeposition (i.e., in floodplains, lakes, estuaries). A goodexample is the 1980 eruption of Mount St Helens volcano,USA, where annual suspended sediment yields after theeruption were as much as 500 times greater than typicalbackground levels. Even 20 years after the eruption, averagesuspendedsedimentyieldsfromthedebris-avalanchedepositsremained 100 times (10