<p>Meandering is one of the most common morphological pattern through which rivers manifest themselves. Here, the attention is devoted to meandering streams carving their path through permafrost floodplains, which typically characterize cold environments such as the Arctic. Despite meandering rivers have been widely studied in the last fifty years, little is known about the dynamics of streams where banks are composed of perennially frozen material. It is inquired whether there is a morphological signature in the planform of permafrost streams potentially deriving from specific thermo-mechanical processes occurring in Arctic landscapes, like the formation of thermo-erosional niches and sediment slumps caused by thaw-weakened soil. To this aim, a bend scale analysis of the planform geometry of several Arctic streams by means of Landsat satellite imagery is employed. Morphodynamic features such as lateral migration rates, channel curvatures, and width variations, are extracted from multispectral remotely sensed data by combining Google Earth Engine (GEE) with an established process-based software (PyRIS).&#160; Following a methodology based on continuous wavelet transform, a set of metrics quantitatively defining the meander shape, which include fattening and skewing coefficients, are used to compare permafrost streams with a series of natural meandering rivers from tropical and temperate regions obtained from the literature. The present analysis opens the way to a systematic integration between remote sensing and physically-based morphodynamic models able to incorporate thermo-mechanical processes uniquely related to permafrost environments.</p>
<p>Climate change is already altering the hydrological regime of Arctic rivers. However, still little is known about fluvial morphological processes and trajectories in permafrost environments. In such iced floodplains, both hydrological and thermal regimes affect sediment transport and riverine morphological processes. Remote sensing represents a powerful approach to investigate fluvial systems in those isolated areas. Nevertheless, its application presents challenges linked to ice seasonality and the limited time window of the morphological activity, alongside the complex permafrost/river spatial patterns and related spectral signatures, which imply significant computational efforts. Addressing this, we propose an improved integration of existing tools for the spatio-temporal extraction of fluvial morphological indicators, combining in a unique working environment the cloud computing capability of Google Earth Engine (GEE) and a process-based tool for riverine multitemporal planform analysis (PyRIS). Fluvial morphological metrics have been extracted from a set of meandering rivers in the Arctic region, outlining the potential of anisotropic image filtering and image segmentation to enhance active channel detection in complex spatial-pattern areas. A 20-40% refinement in small object removal in river mask detection emerges. The synergy among existing instruments enhances the observation of natural river systems in permafrost environments, setting the basis for further studies on morphological processes and the evolution of such pristine and climatically-sensitive river systems.</p>
<p>Besides their environmental values, near-natural rivers offer the opportunity to observe and investigate riverine processes as they would occur under limited anthropic pressures, representing fundamental references for river management and restoration. Even so, few large near-natural rivers can still be found in Europe and worldwide, and their knowledge is often scarce due to a lack of hydromorphological monitoring and baseline studies. Among them, the Vjosa/Aoos River (GR, AL) has been recently recognized as a key large fluvial corridor and a significant model ecosystem. We investigate the catchment-scale recent morphological trajectories of the Vjosa river and its tributaries, coupling the reconstruction of channel adjustments over the past 50 years from remote sensing images with the analysis of possible drivers of change at the catchment and reach scale. We considered eight reaches in the main course of the Vjosa river as well as in some major tributaries (Sarandaporo, Drinos, Shushica) with different morphologies and confinement degrees. Our results underline the sensitivity of the Vjosa system to both hydrological alterations and human pressures. Specifically, it is possible to observe a response&#160; of the system passing from an intense period of high magnitude, frequency, and duration of flood events in the 1960s to a drier period in the following decades. To study the morphological response, three time periods are considered: 1968-1985, 1985-2000 and 2005-2020. In the first examined decades, river trajectories highlight the narrowing of the active channel as a primary response to the hydrological change in the majority of selected reaches, with a 20-50% active width reduction with respect to 1968. In the following time periods, the narrowing rate decreases at the catchment scale, while in the last phase the effect of human pressures in some reaches can be observed. Indeed, from the late 1980s, human pressures at different spatial and temporal scales can be identified, locally altering the natural trajectory of the affected reaches. Such pressures include sediment mining and extensive bank protection of the lowland reaches, together with flow regime alteration occurring in one headwater sub-catchment.&#160; However, our analysis reveals primarily a high sensitivity of the Vjosa system to recent climatic variations, suggesting the importance of accounting for future projected changes in rainfall regime in shaping morphological trajectories. The baseline knowledge on the morphological sensitivity and recovery time developed in this work provides an important reference for the management of highly dynamic river corridors in temperate and Mediterranean climates.</p>
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