# 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  
By considering the subareas, identified as described in Section 2.3, the study reach was divided into polygons of almost equal length, as shown in Figure 4. In particular, eight polygons divided by nine transients (T1-T9), spaced of 330 m in average, were identified. From Figure 4 it can be observed that, during the examined time period, the variations of the external boundaries of the examined channel reach are remarkably little. This means that, despite the high dynamism observed from Figure 2, the active area remains almost constant in time. This is in accordance with previous findings obtained by [PERSON] et al. (2005) which analyzed the dynamics of a reach of the Tagliamento river 16 km long, which includes the reach considered in the present work, in a time period between years 1944 and 2005. By combining information taken from the available images, from the available field photos and from the literature [8; 34], the vegetated islands were classified into three classes: (i) class A, which includes trees with height 2 m \(\leq\)_lv_\(\leq\) 20 m; (ii) class B, which includes buses (herbaceous and low shrubs) with height 80 cm \(\leq\)_lv_\(\leq\) 2 m and mutual distance less than 5 m; (iii) class C, which includes buses with mutual distance greater than 5 m (sparse vegetation). Such a classification is also in line with that adopted by [PERSON] et al. [34] distinguishing tall trees with height >10 m from high shrubs and trees with medium height variable in the range 1.5 m-10 m and from herbaceous and low trees with height < 1.5 m. Figure 5 reports, for each polygon, the extension area of each class of the vegetated islands. For simplicity of exposure, in this figure only the images taken in June 2003, in June 2005 and in February 2012 have been considered. Furthermore, by excluding the islands with sparse vegetation, Table 1 reports, for each polygon, the percent area occupied by classes A (trees) and B (dense buses) with respect to the total active area. As Table 1 shows, in June 2003 the percent area occupied by the vegetation class A tends to decrease slightly as one passes from polygon 1 to polygon 4 and then it increases until reaching polygon 8 where it assumes a value of 2%; the percent area occupied by the vegetation class B is almost equal (around 4%) in all the polygons with the exception in polygons 1 and 6; in June 2005, the percent area occupied by the vegetation class A is almost  
Figure 4: External boundary of all considered images and external limit, polygons and transients. Figure 5: Area extension of the vegetation islands: (**a**) Class A; (**b**) Class B; (**c**) Class C.  
the same as that observed in June 2003 and the percent area occupied by vegetation class B generally increases, with exception of polygons 4, 6 and 8 where it slightly decreases; in February 2012, the percent area occupied by the vegetation class A slightly increases with exception of the polygons 5 and 6 where it decreases; the percent area occupied by vegetation class B strongly increases, assuming values ranging from 8% (in the polygon 6) to 46 % (in the polygon 1). Thus, it can be summarized that, in the time period 2003-2005 (that is characterized by low-magnitude events with some sporadic high peaks), the extension of vegetation islands of class A (trees) slightly increases over the time; the extension of vegetation islands of class B (dense bushes) also increases passing the time with exception of polygons 5 and 6 where an opposite trend has been observed. This behavior is consistent with the fact that, as observed by [PERSON] et al. (2012), the dominant tree species resist to the sporadic flow peaks. This behavior indicates that these vegetated species could have a crucial role on the stabilization or transport of sediments.