Abstract

More than twenty large reservoirs (>20 hm(3)) are located along the latitudinal range of Chile and, so far, only a few investigations have been carried out into the geomorphological effects of reservoirs on this region. This research examines the morphological changes that occurred within the active channel downstream of two dams in gravel-bed rivers under contrasting climates in Chile: The Elqui River in the semiarid north of Chile and the Biobio River in the humid south. The former has been regulated by the Puclaro reservoir since 1999 and is used for irrigation, while the latter is regulated by three reservoirs: Pangue (closed in 1996), Ralco (closed in 2004), and Angostura (closed in 2014), and is used for hydropower generation. The morphological changes were evaluated using satellite images, in a 16 km-long segment on the Elqui River and an 88 km-long segment on the Biobio River. Analyses were carried out to quantify the changes in the morphological units (bars, islands), channel patterns, and flow magnitude and frequency alterations. The results show a 15% reduction in the magnitude of Q(2) flows in the Elqui River and a 64% reduction in the Biobio River. The active channel areas were reduced by 58% in the Elqui (1994-2011) and by 26% in the Biobio (1994-2018). In both rivers, the active depositional areas (central bars) practically disappeared, which led to a simplification of the channel pattern. The main differences, in terms of geomorphological changes, were observed in the areas covered by vegetation; while in the Elqui, a decrease of 87% was detected and, in the Biobio, an increase of 20% was observed. Results support the working hypothesis, showing a higher intensity of morphological changes in the river of the semiarid zone, in line with the hydrological alteration identified in previous research. This research represents the first case study on the geomorphological effects of reservoirs on climatically contrasted Chilean rivers. These analyses provide relevant information to improve the understanding of these effects, as well as provide information on the undisturbed state of river systems that could serve as a reference for restoring and mitigating future river regulation projects.