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 establishment 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 contributed 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 differences 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.