# Investigation of a Gravel-Bed River's Pattern Changes: Insights from Satellite Images
## 4 Discussion  
The present analysis has highlighted that different evolution patterns occur between the time period characterized by frequent low-magnitude events with sporadic peaks (i.e., the time period 2003-2005) and the time period characterized by frequent high-magnitude events (i.e., the time period 2005-2012). In particular, for the examined river reach, in the first case (time period 2003-2005) active multiple threads flowing channels form on the left side, the vegetation cover extends on the right side and the transported sediments are deposited in the central part; in the second case (i.e., during the time period 2005-2012) sediment and woody debris are transported and then deposited after the events facilitating the germination of seeds and the formation of vegetation islands. This is consistent with previous literature works, see as an example [8]. Such a behavior tends to suppress the secondary channels and the main channel tends to assume a single tread pattern. Thus, the different frequency of occurrence of the high-magnitude events could affect differently the sediment dynamics and the distribution and growth of vegetation, determining a different river's morphological behavior. This result is in line with previous findings indicating either that the channel pattern changes especially depend on the channel's sensitivity to flow discharge variations [56] or that the frequency of occurrence of high-magnitude events could strongly affect the degradation processes of the vegetation especially during its germination phase [18]. Thus, the results obtained in this work have demonstrated that, although according to literature [34,43] the hydrological processes are not the only cause of the river's morphodynamic evolution, they exert a significant role in river's dynamics. This is because the frequency of occurrence of high-magnitude events affects, in turn, other factors such as the vegetation distribution and growth and the transport and the deposition of sediments. For the considered case, the fact that the extension of vegetation islands increases always on the right side determines a vegetation-dominated area where the flow resistance assumes high values. This means that in such vegetation-dominated area the sediment fluxes are very low and the sediments are stabilized by the vegetation root systems. On the contrary, the left side is dominated by floods disturbance and is characterized almost by the absence of vegetation, high sediment fluxes and the presence of migrating morphological  
Figure 9: Comparisons between the tracers in February 2012 (in black) and December 2015 (in grey) (images obtained from Google Earth, Image ©2015 DigitalGlobe). features (bars). Thus, in accordance with previous works [37; 43], the vegetation cover determines a sort of interface between the vegetated-dominated area, where the flow resistance is significant, and the flood-disturbance-dominated area, where the action by flow is significant. During the time period characterized by frequent high-magnitude events, parts of vegetation are cut by floods; after the floods, the transported sediments and the woody debris are deposited on the bed, facilitating the germination of vegetation on one side of the study reach, forcing the suppression of the secondary channels and the consequent formation of a single tread channel pattern and, then, of a high-amplitude bend. This morphological feature is characterized by a lateral expansion which prevails over the downstream migration. The different morphological behavior observed during the time periods characterized by different frequency of high-magnitude events could be also related to the different sediment transport mechanism induced by the events. In fact, during floods both coarse material and fine fractions (sand) could be transported and the interactions between the coarse and the fine fractions could affect the bed-load transport and the associated migrating behavior.