Step pool evolution in an alpine torrent

In an instrumented, small catchment of the Dolomites several geomorphic and hydraulic parameters are measured. The fluviomorphological structure of this high gradient stream (the Rio Cordon) is dominated by step pool sequences alternating with several lateral bar reaches and some riffle pool reaches. A comparison of channel morphology measured before and after ordinary and exceptional events – associated with flood characteristics in terms of peak discharge and bedload transport - has been analyzed. The step pool evolution shows that their maximum hydraulic resistance assessment, envisaged by Whittaker & Jaeggi (1982) and suggested by Abrahams et al. (1995), may be obscured by a high sediment supply condition. The main destruction and transformation mechanisms of the steps are also discussed: scouring processes and downstream migration are the dominant features.