This is the first comprehensive long-term analysis of Iceland’s streamflow trends, setting a benchmark for future research. The study reveals a surprising shift: while Iceland’s glacial river flows increased over 50 years, they declined over the past 30, challenging assumptions about meltwater contributions. With Iceland’s hydropower reliant on streamflow, these findings raise concerns about future energy security and water management. As climate change disrupts freshwater systems, this study provides crucial insights into shifting hydrology in high-latitude regions.