# Investigation of a Gravel-Bed River's Pattern Changes: Insights from Satellite Images
## 3 Analysis and Results
### Evolution of the Active Area and of the Vegetation Islands
During the time period 2005-2012 (i.e., period with frequent high-magnitude events), the extension of vegetation islands of class A tends to increase slightly in all the polygons (polygons 1, 2, 3, 6, 7, 8) with exception of the central ones (polygons 4, 5) where it decreases. This could be related to the deposition of fine sediments in some polygons (polygons 1, 2, 3, 6, 7, 8) contributing to the vegetation growing and to the soil reinforcement by roots. In this case a vegetation-dominated area forms. In the central polygons (4, 5) the vegetation is eliminated by the frequent high-magnitude events occurring especially during years 2008, 2009 and 2010. This central area is dominated by flooding disturbances. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **N. polygon** & **Class A** & **Class B** \\ \hline  & & \% & \% \\ \hline June 2003 & 1 & 0.008 & 0.006 \\  & 2 & 0.001 & 0.039 \\  & 3 & 0.001 & 0.039 \\  & 4 & 0.000 & 0.035 \\  & 5 & 0.006 & 0.044 \\  & 6 & 0.010 & 0.185 \\  & 7 & 0.014 & 0.043 \\  & 8 & 0.020 & 0.040 \\ \hline  & **N. polygon** & **Class A** & **Class B** \\ \hline  & & \% & \% \\ June 2005 & 1 & 0.009 & 0.008 \\  & 2 & 0.001 & 0.111 \\  & 3 & 0.000 & 0.062 \\  & 4 & 0.000 & 0.011 \\  & 5 & 0.006 & 0.117 \\  & 6 & 0.009 & 0.115 \\  & 7 & 0.014 & 0.060 \\  & 8 & 0.025 & 0.018 \\ \hline  & **N. polygon** & **Class A** & **Class B** \\ \hline  & & \% & \% \\ \hline February 2012 & 1 & 0.024 & 0.275 \\  & 2 & 0.006 & 0.445 \\  & 3 & 0.001 & 0.466 \\  & 4 & 0.000 & 0.116 \\  & 5 & 0.000 & 0.237 \\  & 6 & 0.023 & 0.081 \\  & 7 & 0.020 & 0.124 \\  & 8 & 0.032 & 0.181 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Percent area occupied by vegetation types A (trees) and B (dense bushes). Thus, the aforementioned behavior confirms that frequent flooding events remove vegetation before colonization can take place while during periods with infrequent flooding events vegetation grows [37]. Figure 6 reports the total area occupied by vegetation of classes A and B along the considered river reach in June 2003, in June 2005 and in February 2012. It can be seen that passing from year 2003 to year 2005 the total area occupied by both classes of vegetation does not change a lot; passing from year 2005 to year 2012 the total area occupied by vegetation strongly increases, especially for the vegetation class B. It should be considered that the increasing trend occurring in Figure 6 between years 2005 and 2012 has been also verified by examining the available field photos. On one hand, the observed behavior suggests that the high frequency of high-magnitude events contributes to the variation of the typology of the vegetation characteristics; on the other hand, it confirms that the flood events could contribute to the formation of islands where vegetation seeds germinate. As an example, from the available photos it is clear (in the year 2008) the accumulation of woody debris and of uprooted vegetation around bushes or trees which could be transported during the previous high-magnitude events. During the period characterized by low frequency of flood events, the resistance of vegetated patches contributes to stabilize the sediment and plants germination; the ability of these plants to resist to uprooting allows them to survive and colonize alluvial environments. Figure 7 shows the vegetation cover (for classes A and B) and the active channels identified from the images taken in June 2003, in June 2005 and in February 2012. From this figure, it can be clearly seen the difference both in the vegetation cover and in the stream pattern passing from June 2003 to February 2012. In particular, it can be observed that the extension of the vegetation cover always increases on the right side of the study reach and the sediment deposition especially on the left side. The vegetation cover determines a sort of interface between the vegetated-dominated area, where the flow resistance is more significant, and the floods-disturbances-dominated area, where the action due to flow is more significant.