Investigating the colonization of river bars by pioneer vegetation, we discuss how the balance between uprooting by flooding events and reinforcement of plant roots determines the ability for vegetation to colonize the riverbed. In order to access the competition between growth and uprooting, we work in a regime where the arrival time of flooding events is comparable with the timescales associated with vegetation, i.e., plant germination and development rates. We present new results for a flume experiment using converging channel walls and draw contrasts with previously published results for a simple channel with parallel sides. This comparison highlights the role of varying local stream power in these processes. We collected the statistics of vegetation root length, number of roots and stem height for both riverbed and uprooted vegetation and show using a stochastic approach how the statistics for vegetation surviving in the riverbed alter a flood result from both the distribution of riverbed biomass before the flood and the distribution of biomass that is uprooted by the flood. Finally, we compare our laboratory results with data obtained in a field experiment to show the similarities in the uprooting processes at both scales. (C) 2012 Elsevier B.V. All rights reserved.