Complex instability of the headwaters of the Aguas Zarcas River

Abstract

In July 2023, a significant landslide occurred in the Pozo Verde sector of Juan Castro Blanco National Park, triggering a series of mudflows and debris that affected the population of Aguas Zarcas in various ways. This article integrates geological observations, historical data collected from oral accounts, mineral identification through diffractometry, and the geomorphological photointerpretation of aerial photographs from 1981. These analyses show that this is not the first time a major landslide has occurred in the area. Oral accounts indicate that the affected area had already shown signs of instability since the 1970s and 1980s. Additionally, photointerpretation of the aerial photographs analyzed in the Geomatics Laboratory of the Central American School of Geology at the University of Costa Rica (UCR) reveals a landscape characteristic of slope instability. The area directly impacted by the landslide shows scars from previous landslides, and three independent bodies of instability were identified. Along the Aguas Zarcas and Barroso rivers, evidence of debris flow deposits from various small landslides, still present in 1981, was observed. Furthermore, the diffractometric analysis carried out at the UCR School of Chemistry on soil samples collected from the landslide yielded interesting mineralogical results related to hydrothermally altered clay groups, including montmorillonite, an expansive clay, although the expansion percentage was not determined. All these aspects together indicate that the headwaters of the Aguas Zarcas River have been a site susceptible to large landslides, making it crucial to maintain constant monitoring of the area.