Limits to fluvial planimetric adjustment imposed by anthropogenic confinement

• Degree of anthropogenic confinement of the Po River is quantified in time and space. • Confinement increases through time and downstream across segments from 20% to 61% • Planimetric sensitivity to increased sediment supply is controlled by confinement. • Anthropogenic confinement > 40 % hampers channel widening at increased sediment supply. • A major hydropower plant delays recovery to quasi-equilibrium state by four decades. The ability of a river to respond to sedimentary perturbations relates to the typology and degree of confinement, which dictates its potential for lateral migration. Over the past century, large rivers worldwide have become increasingly confined by rigid anthropogenic structures as a result of land reclamation and urbanization. However, owing to limited quantitative characterization of anthropogenic confinement, the inherent effects on fluvial adjustment have remained poorly constrained. In the Po River, Italy, historical anthropogenic changes are responsible for profound simplification of channel typology. These changes have left a fluvial mainstem dominantly bounded by levees, groins and similar structures that limit channel migration, thus lateral sediment supply. The planimetric evolution of the Po River mainstem is examined through multi-temporal (1954–2021) mapping of the active channel bed including the secondary channels. These variates are analyzed with respect to the concurrent degree of anthropogenic confinement across six valley segments. To quantify anthropogenic confinement, we map the active channel bed on six sequential airphoto sets and historical topographic maps, and then classify the relevant margins into natural (i.e., mobile) and artificial (i.e., immobile/confining) ones. This analysis reveals that: (i) anthropogenic confinement has increased across segments by a varying degree that ranges between 10 % and 21 %; (ii) that the current confinement increases downstream from 20 % to 61 %; and (iii) that a complex pattern of historical adjustment exists and is primarily controlled by confinement. Following mining collapse in the 1980 s, proximal segments S1-S3 with confinement < 33 % were able to adjust through widening (i.e., rebounding) and reactivation of secondary channels to conditions of increased sediment supply originating upstream and along the adjoining tributaries. Conversely, distal segments S4-S6 with confinement approaching a threshold of 45 % (and greater) have remained insensitive to such changes and continued to narrow, despite concurrent severe hydrological forcing. More locally, this lack of sensitivity is enhanced by the hydroelectric Isola Serafini complex, which today still forces some downstream bed degradation over about 40 km. At the (finer) reach scale, representing the rebound ratio (RR) – a dimensionless metric of planimetric recovery past the 1980 s historical low – as a function of confinement affords definition of a more robust threshold, which drops slightly to about 40 %, and enables to illuminate confluence and barrier effects. This work highlights the value of quantifying anthropogenic confinement, to complement current awareness on river fragmentation and barrier removal towards improved management solutions in the Anthropocene.