Step pool evolution in an Alpine torrent

The effectiveness of a flood as ‘a destructive event depends on the force exerted, the return period of the event, and upon the magnitude of the constructive or restorative processes which occur in the intervening period’ (Wolman & Gerson 1978). This idea of formative events, punctuated by periods of evolution, recovery or even temporary periods of steady state conditions, was used in order to point out the step pool morphology evolution of the Rio Cordon streambed in the 1986–1999 period. The analysis of the characteristics of both floods and coarse bedload recorded at the Rio Cordon experimental station and detailed field surveys of the step pool structure carried out before and after the September ‘94 and October ‘98 floods have served to illustrate the effect of these floods on step pool changes. Floods were grouped into two distinct categories. The first includes ‘ordinary’ events which are characterised by peak discharges with a return time of 1–5 years (1.8–5.3