Woody Debris: Effects on Channel Processes and Form in An Australian Desert Stream

Woody debris accumulation has been primarily studied in stream of the humid zone, and especially in perennial streams. Less is known about the effects of woody debris on channel processes and channel form in ephemeral dryland streams. However, where trees and woody shrubs are associated with ephemeral stream channels, an association that is quite common in drylands where the distribution of trees is largely contracted to fluvial corridors owing to water scarcity in the wider landscape, significant effects arise.

Fowlers Creek is a significant regional ephemeral drainage system arising in the Barrier Ranges north of Broken Hill, in far western New South Wales, Australia. The channel is dry for most of the year, but occasional floods recharge water stores in channel-margin sediments. Consequently, the channel, like many other Australian ephemeral streams, supports riparian corridors of large trees (Eucalyptus camaldulensis, the river red gum) as well as tree growth on the stream bed itself.

River red gums drop many branches to the stream bed during dry weather, and this woody debris is swept up by flood flows, to lodge in large accumulations of detritus, often trapped on the upstream side of channel-bed trees. In some cases, gaps in the woody debris barriers are plugged by accumulations of fallen leaves and twigs, and by mud. These barriers may extend across the entire width of the channel, and in such cases, cause upstream impoundments where flow speeds are reduced and muds settle from suspension to clog the pore spaces within channel-margin sediments. Surficial mud drapes are also set down. These sediment clogs and mud drapes then reduce the rate of seepage loss that would otherwise occur in the impoundments. Elsewhere, the debris barriers generate significant scour and fill, or deflect erosive currents against the bank, triggering channel realignment.