The geomorphological and ecological functioning of the Silala River

Abstract

Perennial rivers in desert regions are relevant environments that tend to concentrate most ecosystem services and values, but are not as studied as much as intermittent and ephemeral rivers. This article reports on the main morphological and ecological features of the Silala River, a narrow permanent river in the arid region of Antofagasta, in the north of Chile. The study was motivated by the dispute between Chile and Bolivia concerning the status of the Silala as an international watercourse. Field surveys of sediment transport using traps and marked sediments (colored and tagged clasts) revealed that finer sediments are moved in higher percentages and for longer distances than coarser sediment fractions, which corroborates the size‐selective nature of sediment dynamics in the river. The stream features the typical step/pool‐plane bed morphology that is to be expected in an alluvial river with the given boundary conditions of slope, grain size of sediments, and lateral confinement. Furthermore, the Morphological Quality Index (MQI) classifies the river as featuring overall good morphological conditions and the river hosts an abundant population of rainbow trout (Oncorhynchus mykiss). Although this is a non‐native species, their presence shows that the general conditions of the river (availability of food and presence of aquatic habitats) are able to sustain a population of resident fish. Overall, the morphological forms, geomorphic processes, and ecological features of the Silala River are typical of an alluvial stream with a permanent flow regime.This article is categorized under: Science of Water > Water and Environmental Change Human Water > Rights to Water Science of Water > Hydrological Processes Water and Life > Nature of Freshwater Ecosystems

Introduction to the Silala River and its hydrology

“…These data were typically collected at 15-min intervals. In addition, in the context of a geomorphological study, Mao (2017Mao ( , 2024 performed temporary measurements of streamflow and temperature (between September 2016 and January 2017) in a river reach 1500 m long, between a location 100 m downstream of FCAB house and 50 m upstream of Weir 9 (Figure 5a). Figure 11 shows the average daily flows of FCAB weirs located in Chile until October 2021, together with the precipitation recorded at the DGA Quebrada Negra station (Figure 10).…”

“…These stations belong to the Pontifical Catholic University of Chile (data fromMao, 2017Mao, , 2024Muñoz et al, 2017;Muñoz & Su arez, 2019;. Abbreviations: AE, actual evapotranspiration; AP, atmospheric pressure; FM, fish monitoring; FMC, fluvial morphological characteristics; NR, net radiation; PP, precipitation; RH, relative humidity; SHF, soil heat flux; SM, soil moisture; ST, soil temperature; STr, sediment transport; WS, wind speed; WT, water temperature.…”

Understanding the Silala River—Scientific insights from the dispute over the status and use of the waters of the Silala (Chile v. Bolivia)

“…Thus, these two papers, while providing a definitive history of the basin and region, also clearly demonstrate that the river has flowed from what is now Bolivia to what is now Chile, along its current path, for more than 8000 years, and indeed that earlier, Pleistocene, landscapes would have generated fluvial flows from Bolivia to Chile. In "The geomorphological and ecological functioning of the Silala River," Mao (2024) reviews the current geomorphological status of the river, based on field research that is pioneering for the groundwater-fed catchments of the Altiplano. This research included sediment labeling and tracking, as well as fish and habitat surveys.…”

Holocene sedimentary history of the Silala River (Antofagasta Region, Chile)

“…Current groundwater levels in the alluvial aquifer have been found to be about 6 m below the current bed level in places (ARCADIS, 2017), which is consistent with this current incision. As flows in the river have remained constant over the last 20 years, however, and the river has been found to have a stable armored bed and be actively transporting sediment (Mao, 2024), the current groundwater levels in the alluvial aquifer are likely associated with longer-term climatic changes, perhaps on centennial timescales. Overall, the river continues to be geomorphologically active and the processes that formed the recent terraces are active today.…”