# Investigation of a Gravel-Bed River's Pattern Changes: Insights from Satellite Images
## 3 Analysis and Results
### Morphological Features and Their Dynamics  
In order to examine the morphological features along the examined river reach at different times, for each image the axis both of the flowing and nonflowing channels were traced identifying the islands surrounded by these tracers.  
As an example, Figure 8 reports the tracers and the islands identified in June 2003 and in June 2005 (i.e., two images of class I) and in May 2011 and in February 2012 (i.e., two images of class II).  
By inspecting the images of class I, a sequence of island features, schematized as bars, has been identified. These features migrate downstream. As an example, by considering the reference point R1 of Figure 8, it can be observed that in June 2003 it is located inside polygon 5 at a distance of around 150 m from the transient T6, but in June 2005 it is found at a distance of around 100 m downstream of transient T7. This means that the point R1 moves downstream of around 580 m over two years. On the left side of the examined reach, a low curvature bend (in red in Figure 8) seems to appear over-imposed to the aforementioned features. In June 2005 the curvature of this bend is more accentuated than that in June 2003 and it tends to migrate downstream and gradually to expand laterally. As literature indicates [53; 54; 55], this behavior is typical at the early evolution stages of a meander.  
Figure 7: Digitized vegetation cover and active channels: (**a**) June 2003; (**b**) June 2005; (**c**) February 2012.  
Thus, it can be summarized that in the time period between June 2003 and June 2005, which is characterized by frequent low-magnitude events with sporadic high peaks, the active part of the study reach is mainly concentrated on the left side where a sequence of migrating bars and a bend of low curvature form. At this stage, the bars represent the dominant morphological feature.  
By inspecting the images of class II, a stream pattern very different from that observed between June 2003 and June 2005 can be seen. In particular, in May 2011 a meander bend of high curvature is clearly visible. This suggests that passing from June 2005 to May 2011 a significant lateral migration of the bend occurred. Then, passing from May 2011 to February 2012 such a high-curvature bend furtherly expands laterally with no evident downstream migration. As an example, the reference point R2 is found along the transient T4 both in May 2011 and in February 2012. Thus, in such a time period, the meandering bend becomes the dominant morphological feature. According to the literature [53; 54; 55] for a high-amplitude meander wave the lateral expansion prevails over the downstream migration.  
The aforementioned behavior is also confirmed by Figure 9 where the tracers identified in February 2012 are compared with those obtained in December 2015. This comparison shows that the main channel further tends to expand outwards until in December 2015 the bend is also slightly skewed with the formation of chute channels: the reference point R3 moves both outwards and upstream.  
Figure 8: Morphological features and evolution from June 2003 to February 2012 (images obtained from Google Earth, Image ©2015 DigitalGlobe).  
Thus, it can be summarized that while in the time period between June 2003 and June 2005, characterized by frequent low-magnitude events, multiple bars form and the downstream migration is the prevailing movement, in the time period between June 2005 and February 2012, which is characterized by frequent high-magnitude events, the channel tends to assume a single tread pattern with prevalent lateral expansion.