Theoretical population genetics examines how the interplay between demography and dispersal regime shapes genetic diversity along range expansions. Yet it is challenging to test these predictions (such as the surfing phenomenon) in natural populations, in part because of the high stochasticity inherent to the realisation of each colonisation event. Here, we benefit from a remarkable ecological system to disentangle colonization patterns thanks to annual biological replicates1,2. Embanked in the Alps, the Durance River valley channels the dispersal of a plant pathogen, the poplar rust fungus. Each year, this species spreads out clonally along the riverbanks. We monitored the epidemics and inferred that dispersal of this pathogen results from a fat-tail kernel. We subsequently tested how this dispersal regime leads to a conservation of genetic diversity, as expected theoretically. We observe clones that play leapfrog from site to site, which prevents a single genotype to surf along the river. Notably, whatever the annual replicate, the range expansion can be split into two phases with different genetic outcomes. Upstream, fast colonization maintained high genetic diversity. Downstream, the colonization wave progressively faltered, diversity eroded, and differentiation increased, as expected under recurrent founder events. Yet the magnitude of these changes differs from year to year as epidemics differ in intensity. We hence provide, through the study of this remarkable ecological system, direct evidence of the interplay between drift and migration at work.