Vegetation debris dams occur on average every 27 m of channel in a drainage basin in the New Forest, Hampshire, England, and within less than 12 months 36% changed position or were destroyed and 36% changed character. Such dams significantly affect the timing of flood peaks as they are routed through the channel network; their significance has been demonstrated by preliminary analysis of hydrograph travel times by measurements in a reach at different flow stages, and by measurements before and after dam clearance. There was a difference in travel time of over 100 minutes for the situation with and without dams for a discharge of 0.1 m3s−1but a difference of only 10 minutes for a discharge of 1.0 m3s−1along the same 4028 m channel reach.