Flood risk can be considerably increased by the presence or transport of Large Wood (LW), e.g. due to bridge clogging by logjams and increased inundation probability. The damage potential of LW depends mainly on the transported volume, but also on the size and shape of individual wood pieces. The present study investigates the fragmentation of entire trees down to smaller LW pieces due to fluvial transport, based on measurements after the 2005 flood event in Switzerland. The length reduction of LW was calculated using an allometric function, assuming that diameter is not changing considerably. Transported LW pieces had a maximum length of 10 m, which was apparently limited by the channel width. The calculated reduction coefficients range between 0.1–0.33, with a mean value of 0.2, which implies that recruited trees are on average reduced to 20% of their original height. The size reduction depends less on the transport distance than on the intensity of the physical degradation processes.