Flood geomorphology of the Katherine Gorge, Northern Territory, Australia

Abstract

The Katherine Gorge is a narrow, deep canyon system developed in resistant sandstone. The tropical monsoonal climate results in rare flood events of very large magnitude. These floods flush sand-sized sediment as washload through the gorge, but local accumulations are preserved as slack-water deposits at tributary mouths. Paleoflood hydrologic analysis of the deposits, employing step-backwater flow modeling, allows quantitative estimates to be made of geomorphically significant flows. For an annual exceedence probability of 10 −2 , the gorge may experience discharges of 6,000 m 3 /s, mean velocities of 7.5 m/s, flow depths of 15 to 45 m, stream powers per unit area as great as 1 × 10 4 watts/m 2 , and bed shear stresses of 1.5 × 10 3 N/m 2 . Such flows are required to transport boulders 3 m or more in intermediate diameter that occur at large-scale riffles. At maximum stage, pools reach depths of 45 m and are preferentially developed at vertical joint intersections in the rock, probably through intense hydraulic action. Pool-and-riffle development thus reflects the channel boundary characteristics for extreme flow conditions. Additional indicators of especially intense flow phenomena include potholes, flutes, abraded facets on rock bar surfaces, and scabland development on upland bedrock surfaces.