Nature and hydro-geomorphic roles of trees and woody debris in a dryland ephemeral stream: Fowlers Creek, arid western New South Wales, Australia

Abstract

Woody debris jams (WDJs) and the rate of wood fall into the channel were observed along an ∼18 km study reach of ephemeral Fowlers Creek. Fifty-five WDJs were recorded, including nearly 700 key large wood pieces. WDJs obstructed on average 31% of the channel width (maximum 98%), and had a mean projected area facing the flow of 9.7 m2. Abundance of WDJs is low in comparison with some humid forested streams, but channel obstruction is significant, owing to the many in-channel trees capable of supporting tall, stable jams of up to 3.3 m height, and the lack of wood decay in the arid conditions. The rate of wood recruitment to the channel was judged by tallying the debris that has accumulated through 6 months of no-flow conditions. Coarse wood recruitment averaged 1 piece/100 m/a or ∼0.03 m3/100 m/a. Again, this is low in comparison with forested catchments, but is comparable to the bottom end of the range of published recruitment rates. WDJ occurrence and hydro-geomorphic significance vary with position along Fowlers Creek, which exhibits channel contraction arising from transmission loss. The link between woody debris and channel form and process in this ephemeral stream is discussed.