As marginal beaches at the edge of deltaic spits are continuously formed, eroded and re-deposited, in a continuous battle between wave and river dominance, the interpretation of these precarious landforms from satellite imagery must be taken with caution. In the case of infrequent satellite overpasses, accretion in an image may only reflect temporary conditions. However, when image archives include frequent overpasses (as in the Sentinel constellation), the occurrence of a recent uninterrupted trend can be considered as a reliable evidence of delta progradation (Fig. 1). This is what we observed for the two Po di Pila mouths examined in this study, in a way contradicting previous works that describe the delta still in disastrous conditions regarding the sediment feeding by the river3,5.

Deltas are complex and dynamic landforms, with a vast submerged zone that is rarely visible in traditional air photography and/or surveyed during monitoring, especially its shallowest parts, where land surveys cannot extend and large survey vessels cannot access. Remotely sensed optical data allow detecting the changes of even the shallowest coastal morphologies (between tide levels ~0.8–1.5 m) but in the past had the limitation of a low frequency of data acquisition and an unsatisfactory on-the-ground resolution. The Sentinel platform revisiting capabilities permit the frequent comparison between images captured at the same tidal level and increase the possibility of finding acquisitions made during conditions of good water transparency, as in Fig. 1. The inability of a deeper water penetration prevented us from exactly pinpointing when nearshore aggradation of the submerged landforms has started, triggering the emersion of the mouth bar. However, it is clear that the underwater aggradation had already been developing for some times (few years 2–4) as it can be seen in Fig. 2e around the secondary Pila northern mouth and in Fig. 2f on the main outlet.

Two main factors generally control delta progradation: longshore sediment transport system and the amount of sediment supply by the river. Furthermore, storm-related erosive marine processes can play a fundamental role in contrasting the mouths progradation and aggradation. Previous works10 found no evidence that in the last two to four decades the coastal sediment transport system and the wave activity have changed. As it can be seen in Table 1, in the last decade, the frequency of storms has remained substantially constant, showing only limited deviation within the normal climate variability, also during the years of the fastest coastal progradation (2015–17). This evidence supports the hypothesis that the sediment budget at the Po river mouth has turned positive again propelling coastal accretion at the northern margins of the delta (Pila mouth bars) after a few decades of low yields.

Figure 3 clearly shows that after about 30 years of relatively low potential effective flows, in the 2000–2002 interval the situation was reversed and higher transfer rates of bed sediment to the mouth were likely resumed. It is worth noticing that this is not the only factor that could have contributed to restore a substantial sediment flux to the Po river mouths. In fact, the prohibition of bed material harvesting, defined by law in the late 1980s, could be showing now its positive effects. The sediment infilling of the very many (thousands) weirs in the headwaters may have contributed as well. They were constructed during the last century all across the mountainous and hilly areas of the Po catchment, as torrent control works to contrast slope degradation and river incision. Nowadays most of these weirs have already fulfilled their original purpose and are completely silted-up. With no more sediment being trapped behind them, they no longer hinder the natural transfer of sediment from the catchment slopes to the river network. Moreover, climate change may have played an important role in terms of accelerated glacier and snow melting in the Alps, which increased the spring season discharges of the Po4, resulting in higher values of the potential effective discharge (Q k ) (Fig. 3). In Fig. 3 also the five-year mobile average has been included in order to point out prolonged periods with higher values of Q k which may reflect a substantial bedload supply, more effective in contributing to the formation of mouth bars.

In conclusion, the results of this study point out that the Po delta, after a sediment budget crisis extended throughout the first years of the 21st century, has started again to prograde. In the most recent years (>2010) between the Pila mouths the process has been accelerating and now some progradation is taking place as showed by the growth of mouth bars in Fig. 1. This signal cannot be extrapolated to the whole of the delta, as it is relative to only a portion of it. It is probably necessary to consider a longer time span in the future to assess the response of the whole system.

The current trend marks a countertendency compared to many other world deltas5,6 and can be summarized as the result of sediment management policy paired by unexpected climate change positive feedbacks (e.g. accelerated spring ice/snow melting). In any case the area still remains highly vulnerable to sea-level rise as well as storm and river flooding due to the low elevation above mean seal level.