In populous, water-limited regions, humans have profoundly altered the river and floodplain environment to satisfy society’s demands for water, power, navigation and safety. River management also profoundly alters riparian forests, which respond to changes in disturbance regimes and sediment dynamics. In parallel studies, we compare forest and floodplain development along two of the most heavily modified rivers in mediterranean-climate regions, the middle Sacramento (California, USA) and the lower Rhône (SE France). The Sacramento was dammed in 1942 and is now managed for irrigation, hydropower and flood control. The Rhône channel was channelized for navigation prior to 1900, and since then has been dammed and diverted at 18 sites for hydropower and irrigation. We conducted extensive riparian forest inventories and sampled fine sediment depth in regulated reaches within both systems, and compared pre- versus post-dam patterns of deposition and linked forest development.  We sampled 441 plots (500 m² each) along 160 km of the Sacramento, and 88 plots (1256 m²) stratified by management epoch (pre-channelization, pre-dam, post-dam) along 160 km of the Rhône.

On the Sacramento, forest composition showed shifting tree species dominance across a chronosequence of aerial photo dates over 110 years. The transition from willow to cottonwood (Populus) occurred within 20 years, and the transition to mixed forest started after 50-60 years. On the lower Rhône, 15% of the current riparian forest occupies old floodplain land pre-dating the 19^th century riverbank engineering projects, 38% occupies post-channelization and pre-dam surfaces, and 47% colonized land that emerged after dam construction and reduction in river base flows between the 1950s and 1980s. Compared to younger, post-dam forests, the pre-dam stands occupy higher geomorphic surfaces, are more species rich, and have a much more extensive shrub and vine stratum (20-50% cover overall). The shift from Populus dominance to other species began approximately a decade earlier on the Rhône compared to the Sacramento. Both rivers showed a strong understory presence on young floodplains by Acer negundo (box elder), which is non-native and invasive in Europe, suggesting similar processes of colonization and propagation in both systems. Overall, the Sacramento can serve as a predictive reference system for the Rhône, where significant restoration efforts are underway to improve riparian structure and function in diverted river reaches.