Waterhole form and process in the anastomosing channel system of Cooper Creek, Australia

One of the most distinctive features of Cooper Creek's anastomosing channel system is the preponderance of waterholes, enlarged segments of channel ra nging in length from 100 m to over 20 km. Enlargement occurs in both the wi dth and depth dimensions to give bankfull cross-sectional areas four to eig ht times the average size of individual anastomosing channels. The anastomo sing system is cut into a floodplain of subtle relief but depths of incisio n can be highly variable, giving rise to changes in the degree of anastomos is as different sets of channels are activated during the filling stage of a flood. Also, cross-sectional form and bed topography can vary markedly ov er short channel distances, which has implications for local flow condition s and erosive potential. Hydraulic geometry analyses suggest that cross-sec tional area adjusts much more readily than velocity to increasing discharge in waterholes, particularly if there is limited lateral confinement. Conse quently, velocities tend to be rather modest across a wide spectrum of flow s, with average values rarely reaching 1 m s(-1) Nevertheless, observations made during a 1:20-year flood reveal a deeply penetrative band of relative ly high velocity in one waterhole, suggesting that localized values of bed shear could be quite large even when sectional averages are moderate. Water holes appear to be a product of the present regime, since they concentrate flow from both feeder channels and the floodplain during flood discharges. In several respects they resemble chains-of-ponds morphology [Eyles, R.J., 1977. Birchams Creek: the transition from a chain of ponds to a gully. Aust . Geogr. Stud. 15, 146-157.], and could represent the discontinuous section s of a present-day channel, which cannot be maintained in that form over it s entire length. That the one waterhole to accommodate the entire cross-val ley flow at bankfull stage (Meringhina Waterhole) has cross-sectional dimen sions similar to those predicted by appropriate regime equations lends supp ort to this argument. Essentially, waterholes are self-maintaining scour fe atures, which play a significant hydrological and geomorphological role in the Cooper Creek anastomosing system. They provide long-term water storage in an arid environment, and their fixed position helps to maintain the stab ility of the multi-channel pattern. (C) 2000 Elsevier Science B.V. All righ ts reserved.