Evolutionary trajectory of channel morphology and controlling factors in a large gravel-bed river

Interpretation of channel changes is not always straightforward since it requires a detailed reconstruction of the evolutionary trajectory of channel morphology, a quantitative analysis of controlling factors and, finally, identification of links between evolutionary trajectories and controlling factors. The aim of this paper is to explain channel adjustments and controlling factors in the Tagliamento River, a large gravel-bed river in northeastern Italy. Traditional methods for studying historical channel changes (i.e., use of aerial photos, topographic data) were employed, but numerical modelling turned out to be very useful for the quantification of bedload transport and for supporting interpretation of past changes and likely future channel evolution. River channel underwent three main phases of adjustment over the last 200 years. The first two phases, from the end of the nineteenth century to the early 1990s, were characterized by narrowing (channel width decreased from 1250 to 540 m) and incision (about 1 m); and the third phase, from the 1990s to present day, by widening (from 540 to 600 m) and slight aggradation (about 0.2 m). Combining evolutionary trajectories of channel morphology and analysis of controlling factors we argue that the long-term channel evolution of the Tagliamento River was driven mainly by human intervention at the reach scale (i.e., sediment mining and channelization). Changes in sediment supply in the catchment area had no, or minor, effects in the study reach. The most recent changes (i.e., widening and slight aggradation) are explained as a response to past disturbances that have produced a remarkable change of channel geometry and an increase of unit stream power in the reach. Magnitude and frequency of formative discharges are a key driving factor of the intensity of recent changes, while this was not the case during the previous phases of adjustment.