Incised Channels: Disturbance, Evolution and the Roles of Excess Transport Capacity and Boundary Materials in Controlling Channel Response

Channel incision is part of denudation, drainage-network development, and landscape evolution. Rejuvenation of fluvial networks by channel incision generally leads to further network development and an increase in drainage density as gullies migrate into previously nonincised surfaces. Large anthropogenic disturbances, similar to large or catastrophic natural events, greatly compress timescales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to anthropogenic and natural disturbances are presented for fluvial systems in the mid-continent, Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically controlled sediment (sand) provided by bank erosion. The nonlinear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems. The sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape, with erosion from the channel banks generally the dominant source. Disturbances that either affect the hydraulic forces available to perform geomorphic work or change erosional resistance creating conditions of excess of flow energy can result in incision. Channel incision, therefore, can be considered a quintessential feature of disequilibrated fluvial systems.