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Riverine floodplains are pulsing ecosystems that expand and contract with changing flow. In this study we quantified large-scale expansion and contraction dynamics of surface waters along a 41.5 km braided section of the last remaining semi-natural large Alpine gravel-bed river (Tagliamento River; NE Italy). To assess surface-subsurface exchange patterns We measured discharge and vertical hydraulic gradients at multiple locations along the corridor. We identified two river sections delineated by distinct geomorphic knickpoints. In the upper 29 km, Section I (losing zone), surface flow decreased on average by 2.5 +/- 0.8 m(3) s(-1) per river-km. In the downstream 12.5 km, Section II (gaining zone), surface How increased on average by 0.3 +/- 0.1 m(3)s(-1) per river-km. The losing zone experienced frequent and extensive drying and rewetting cycles. The length of the dry river section was measured over a 1.5 year period using differential GPS. Up to 23 km of Section I fell dry at the surface. Frequent and irregular flow pulses led to rapid expansions of the wetted channel at velocities of up to 3 km h(-1), while the subsequent contraction velocities were less than 0-5 km h(-1). Water level was linearly regressed against the total length of the dry river section (r(2) = 0.74; p < 0.0001). This relationship, in combination with a continuous stage record, was used to evaluate expansion and contraction dynamics over a 4 year period. Timing, frequency, magnitude (spatial extent) and duration of expansion and contraction dynamics reflected the flashy flow regime of the Tagliamento River, including a high intra- and inter-annual variability of surface drying and rewetting. Our study emphasizes that even small changes in How can cause major increases or decreases of ecosystem size, thereby creating a highly dynamic and harsh environment for both terrestrial and aquatic organisms. Copyright (C) 2007 John Wiley & Sons, Ltd.