# Before and After: A Multiscale Remote Sensing Assessment of the Sinop Dam, Mato Grosso, Brazil
## 4 Discussion
Through river connectivity fragmentation, the construction of dams on the tributaries of the large rivers prevents annual migrations of fishes, and keystone species, now no longer able to reach their spawning grounds in the headwaters, have suffered a decline in their populations over time, and may eventually become extinct in the dammed rivers [37, 98]. The resulting lentic zones also leave few stretches of rapids between the dams, with serious consequences for vulnerable rheophilic species, especially sensitive to the loss of connectivity of the river. Our results (Figures 13 and 14) reveal the lentic zone of the HPPS encompasses important areas with previously fast flowing current and white-water rapids inhabited by rheophilespecies. The effects of permanent change of the water level, as well as the reduced flow are further enhanced by the absence of the seasonal flood pulse vital for the breeding cycle for many of the migratory species in the river. The dams on the Teles Pires river eliminate some natural barriers, such as the Sete Quedas waterfall (Teles Pires dam) [99] and species previously restricted to the downstream sector, such as _Pterodoras granulosus_, have been able to establish themselves above the waterfall in the former rapids sector of the river due to the permanent high-water levels. Similar changes in the fish communities are expected within the HPPS lentitic zone. The riverveed plants of the family Podostomaceae also have a life cycle closely tied to the flood pulse and the high current of the rapids. While their vegetative state is submersed, both pollination and fruit formation occur when the plants are entirely out of the water during the dry season. Without the flood pulse and dynamic changes in water levels over the seasons, the plants, vital to many rheophile species, will no longer grow in the lentitic zone [100]. Most UAS can be used in a basic photogrammetry workflow (Figure 8) to acquire the photographs needed to generate an orthomosaic or 3D model (Figures 14c, 15 and 17). Studies such as [29, 101] showed that accurate, fine spatial detail scale SfM products can be achieved at relatively low cost. Mapping aquatic habitat complexity using this technology, especially in the highly variable tropical environments, can help to assess habitat suitability for many species. To do so, several studies (e.g., [22, 25, 27, 28, 102, 103, 104, 105, 106, 107]) document the best practices for acquiring UAS based data and processing options for achieving accurate results in a range of conditions and environments. Some of the common main points for considerations elaborated upon by those studies include:  
* planning for an appropriate GSD (e.g., 1-3 cm is generally sufficient for most fine scale applications),
* understanding the relationship between focal length, sensor size and flight altitude on the expected GSD,
* understanding the importance of photograph quality and target type on the outcome of the SfM workflow (e.g., the white water rapids at Corredeira do Sulpicio cannot be reconstructed),
* utilizing a flight planning application and flight controller software to ensure proper front and side overlap between photographs
* understanding the impact of file type and compression of the photographs on the SfM products  
Across the various commercial and open source software available for the SfM processing, a plethora of studies (e.g., [108, 109, 110, 111, 112, 113, 114, 115]) have compared the impact of software choice, software settings, photograph pre-processing, etc., for both above and below water applications. The SfM products and datasets derived from them may become the only digital reference data to sites that are permanently modified, in this case, the rheophile habitats that no longer exist after inundation (Figures 14c and 15). Data visualization is a strength of remotely sensed data and the derived analytical products.