Reach-scale morphological adjustments and stages of channel evolution: The case of the Trebbia River (northern Italy)

A multitemporal series of aerial photos and cross-section topographic surveys have been used to analyze reach- scale channel evolution along a segment (length of about 22 km) of the lower Trebbia River (Northern Italy) with the aims to investigate the relations between channel width vs. bed-level adjustments and to identify spatio- temporal patterns of stages of channel evolution. Dendrochronology was used to determine the age of tree estab- lishment of riparian and island forests during channel evolution. We identified a first phase of major adjustments (1954–1992) following a series of disturbances, dominated by channel narrowing and bed incision. During the final stage of narrowing, woody vegetation establishment con- tributed to stabilize new floodplain or island surfaces. A period of partial morphological recovery occurred from 1992 to 2010, dominated by an inversion of trend of channel width. During the phase of partial recovery, a stage of widening combined with a continuation of bed incision was identified, and a last stage characterized by widening and initial aggradation was observed on the central portion of the study reaches. Suitability and dif- ferences of existing channel evolution models (CEMs) derived in other geographical contexts were discussed, and a specific conceptual model comprising four stages of channel evolution was developed for the lower Trebbia River.