# Investigation of a Gravel-Bed River's Pattern Changes: Insights from Satellite Images
## 4 Discussion
According to previous literature works [57; 58; 59; 60], the coarse particles slow or stop each other, but the presence of finer particles trapped in their interstices determines that their mobility becomes higher. [PERSON] et al. [61], as an example, indicated that the fine sediments represent a critical element to maintain the rapid bar growth rate and sediment transport controlling the braided rivers. They also suggested that, by limiting the bank erosion rate, the meandering morphology tends to be maintained. It should be also considered that the transfer of sediments from one zone to another varies with both the size of materials and with the discharge. Furthermore, the coarsest materials move less frequently and shorter distances than the finer ones. This is in accordance with [PERSON] et al. [43] which observed that vegetated islands show predominantly sandy surface sediments containing silt, clay and organic material, whereas sparsely-vegetated surfaces of youngest islands show coarser gravel-sand mixtures. After moderate flows the deposition of sediment determines a clear firing of vegetated areas and the presence of shrubs increases the trapping of fine sediments. Results have shown that during the period 2003-2005 (period characterized by low-frequent high-magnitude events) the vegetation cover does not increase a lot. As a consequence, the banks of the active channels are more erodible leading to the channels' widening and the formation of islands in the central part of the river reach. Passing the time, the vegetation species more resistant dominate and the developed islands aggrade and extend laterally. The increasing presence of vegetation on the right side of the river reach, during the successive time period, on one hand increases the hydraulic resistance and on the other hand increases soil strength due to the vegetation roots system. The increase of soil stabilization due to the roots system, in turn, makes the banks resistant to the erosion during the period with frequent high-magnitude events, reducing the number of active channels on the floodplain, maintaining an almost constant width and facilitating the formation of the meandering bend. According with [PERSON] et al. [39], the growing of vegetation in time leads to sediment trapping and transforms the gravel bars to new floodplain surface. The progressive prevailing of lateral migration and the evolution of the bend determines an increase of the curvature and, consequently, of the bend effect. In fact, as flow enters in the bend, because of the channel's curvature, an unbalance between the centrifugal force and the transversal pressure gradient force develops and the secondary flow motion (often called as helical flow motion) evolves along the bend itself [62; 63; 64]. The secondary flow motion, with surface velocity directed towards the outer bank and the near-bed velocity directed towards the inner bank, tends to move the sediment from the outer bank towards the inner bank, thus facilitating the development of the point bar in the apex of the bend. Furthermore, as several studies demonstrate [62; 63], the secondary flow contributes to the redistributions of the primary flow and of the maximum velocity gradients' location along the bend [62; 64; 65]. In turn, the spatial variations of the primary velocity and of the bed shear stress determines also a spatial variation of the sediment fluxes along the bend. Thus, the interaction between the channel curvature effect and the migration movement could explain the development of the bend at late evolution stage [66].