Riparian Forest Development Following Channelization and Dams on Large Rivers: the Role of Management History in Setting Divergent Ecological Pathways

English Abstract:

River management profoundly alters riparian forests, which respond to changes in channel mobility and disturbance regimes imposed by channel engineering, flow regulation and floodplain development. However, the specific impacts to riparian corridors (e.g., forest extent and community dynamics) can differ greatly among rivers, even ones with similar climates and development pressures, because of differences in river management history. In this study, we compared contemporary riparian forest and floodplain development along two of the most heavily modified rivers in the mediterranean-climate biome, the middle Sacramento (California, USA) and the lower Rhône (SE France). In both basins, we set out to understand how river management has affected the composition, sustainability, and restoration potential of the riparian zone at a large spatial scale. We conducted extensive forest inventories and sampled fine sediment depth in regulated, 160-km reaches within both systems, and compared pre- versus post-dam patterns of deposition and linked forest development.

On the Sacramento, where channel migration still occurs and drives new forest development, tree composition shifted across a chronosequence of 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 Rhône, where the river was extensively engineered for navigation and later diverted for hydropower, the pre- versus post-dam surfaces at each site showed less of a shift in species composition, but had more subtle characteristics related to basal area growth and cover of shrubs and vines. Notably, the channelized Rhône had an almost complete absence of small stems of pioneer willows and poplars, portending a long-term dominance by post-pioneer communities in the future. 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.