The study site spans 620 km of Peru’s Marañón River. (A) Regional location of study domain (red outline). (B) The Marañón River study domain including all sampling sites and proposed dam locations to date. Sampled subbasins (orange outlines) represent tributaries sampled for hydrochemistry, whereas the hatched Utcubamba basin is a major unsampled tributary, see Figure 2 for tributary names. Discharge gages are shown as green (operational) and purple (historical). Cellendin, the largest municipality, is also indicated (orange circle). The nearest Global Network of Isotopes in Precipitation (GNIP) precipitation isotope sampling sites are the Marcapomacocha site inland from Lima and the Puerta Almendres site below the Maranon-Ucayali confluence at Iquitos (blue dots on (A)). (C) Continental location of study domain (red outline). Basemap in (C) credit to National Geographic. DOI: https://doi.org/10.1525/elementa.290.f1