Hydrogeomorphological repercussions of anthropic topographical signatures in the north of Portugal: the documented cases of the Abadim and Piscaredo Levadas

The aim of this study is to present how anthropogeomorphological aspects, generated from the construction, renovation and maintenance of channels in northern Portugal, have hydrogeomorphological repercussions on these slopes and valley bottoms. This research was possible from the cross-checking of documentary data found in the historical archive of the Portuguese Environment Agency, and from field activities carried out in the Levada do Rei, in Abadim, and in the Levada de Piscaredo, in Mondim de Basto. The main aspects raised were the cuts in the slopes, the compartmentalization processes of the water volume, and the interactions promoted along the slopes, both by erosive and depositional processes. The documentary survey reveals that the channels are old and date back to the middle ages, having both undergone renovations in the 1970s that promoted the covering by impermeable slabs (rock cuts) and improvements in the aspects of erosive processes management and water flow. In the field it was possible to identify that from the Abadim Levada the water flows can be practically suppressed from the natural channel by the action of the transposition, which is not observed in the Piscaredo channel, since the supply flows from the rivers Cabril, Cabrão and Ribeira Velha are sufficient to maintain the transposed flow and the natural volumes. The study areas, although they are inserted in identical geological context (granites and schists), differ at the level of geomorphology and, especially, in the slope inclination. Thus, the erosive processes are more pronounced along the Piscaredo Levada, where the maintenance works are more incisive in attempts to control these processes, with cuts in the slopes that exceed two and a half meters and dozens of points with road crossings, gutters, dams, and walled sections. For this levada, works were identified as debris passages, collected from the upper half of the slopes, walls to stabilize erosive forms, and dams to collect water in natural water lines. The sediments deposited along the artificial channel generate demands for cleaning, which are maintained by the users and the competent regulatory agency. From this maintenance, the anthropic marginal dikes are reinforced by the sediments, previously transferred from the hillside to the channel. In the Abadim levada, as observed in the field, the smoother slopes considerably reduce the demands for erosive form containment works. The lower interaction between artificial channel and slopes reduces the number of gutters to divert flows from the slopes and also the demands for cleaning, not promoting significant marginal anthropic dikes. The investigation reveals how the interactions between natural processes and anthropic demands generate substantial changes in the landscape, from the shapes of the slopes, both in the cut for opening the channels, as well as the erosive forms and anthropic dikes accommodated in their margins. Considering that the natural fluvial systems are also impacted by dams, sediment retention and alteration of the flow, knowing these aspects in a more detailed way helps to improve the efficiency of these structures, reducing operation costs and water loss.