# Mapping Mountain Landforms and Its Dynamics: Study Cases in Tropical Environments
## 4 Results
### High-Resolution Fluxial Geomorphology of a Humid Tropical and Pristine Reach  
UAV surveying was used to map the fluvial geomorphology of a river reach within the San Lorencito catchment. The reach has a length of 280 m and a slope of 3.27\({}^{\circ}\). This reach is near the main biological station used for hydrometric monitoring of rainfall and streamflow. The channel is characterized by an intercalation of straight single-thread step-pool and cascade-channel morphology. The step-pool repeating sequences are located within the mid-reach area, composed mainly of a jammed accumulation of boulders. The steps can easily be identified, with an increase of turbulence and abrupt elevation change within the channel. Meanwhile, the cascades, characterized by a chaotic arrangement of boulders, can be found on both extremes of the surveyed area. Slope is an important factor when delineating these step-pool/cascade areas; the first round is between 3-7.5% and cascade sections are higher than 7.5%. The elevation profile shown in Figure 6 clearly shows the slope and elevation changes in the reach, allowing the identification of a cascade morphology on the extremes of the survey and a step-pool within the middle section.  
The elevation profile derived from the DEM also allowed the detection and then accumulation of sediment within the channel. Two small alluvial fans with an area ranging between 357 m\({}^{2}\) and 125 m\({}^{2}\) were identified. These small fans would be undetectable by satellite imagery, but the UAV allowed the detection of these small fans formed by hillslope tributaries. These two alluvial fans cause a slight elevation disruption in the channel, forming a step and leading to increased turbulence.  
Another important aspect of the survey was the capability of estimating particle size of the boulders, ranging from an average diameter of 1-2 m to a maximum of 4.7 m. The survey also allowed the mapping of a fluvial island located downstream of the reach. This island has an oval morphology and an estimated area of 184 m\({}^{2}\). The island causes a multi-thread small section, caused by the bifurcation of the stream.  
We present a low-cost method for accurate monitoring of river dynamics. Figure 6 shows in a red rectangle (B), a monitored area for high-resolution channel changes. Slight sediment accumulation and transport dynamics were observed between the two observations. During these two observations, slight changes occurred in the channel; these modifications vary from boulder transport to sand-gravel accretion.  
Figure 7 shows the change in the elevation profile during the monitoring period. The slight elevation variations are evidence of the transport of boulders. The elevation changes ranged from 40 cm to 63 cm. The survey was also capable of detecting a tree fall that exposed fine particle sediments (gravel-sand). This tree was located on the fluvial island described in Figure 6. The turbulence variations are related to a reorganization of the alluvium. An increase in boulders was observed during observation (B). This caused increased turbulence in the channel on the east side (downstream).  
Figure 6: High-resolution fluvial geomorphology of a reach of the San Lorencito River. (**A**) Location of the survey site: the purple dot indicates the change detection location within the San Lorencito catchment, performed by the UAV surveying. (**B**) Geomorphological mapping of a river reach: the survey shows with accuracy the elevation and slope changes within the channel, allowing the identification of step-pool sections and cascades. The purple rectangle indicates the change detection within the San Lorencito river reach.  
Figure 7: Monitoring high-resolution changes in the San Lorencito River. The analysis of both surveys allowed the detection of slight and detailed channel changes; the elevation profile shows how several boulders have been transported during the observation period. (**A**) first observation 14 8 2017 and (**B**) second observation 21 September 2017.