In August 2026, a massive iceberg, roughly the size of Manhattan, broke off from the Petermann Ice Shelf in northern Greenland. This event marked the largest calving—where ice breaks off from glaciers—since 2020. Scientists had been studying the area for years, focusing on a unique feature: the first known ice shelf estuary on the Petermann Ice Shelf. An estuary is a body of water where salt water from the ocean mixes with fresh water from a river. This particular estuary was discovered in 2021, and recent research led by the Cooperative Institute for Research in Environmental Sciences (CIRES) has provided new insights into how it formed and its potential impact on ice stability. The research, published in the journal The Cryosphere, used satellite imagery to measure the rate at which a surface meltwater river carved into the ice shelf, creating the estuary. This process, previously not well understood, revealed that the estuary behaves more complexly than earlier studies suggested. "We were able to measure how quickly the river cut into the ice shelf to form the estuary," said Michela Savignano, a doctoral student at the University of Colorado Boulder and lead author of the study. The findings may help explain the recent calving event, as the repeated formation and disappearance of the estuary could have weakened the ice shelf over time. This research builds on a 2021 study that confirmed the existence of the estuary using high-resolution satellite images. The images showed sea ice floating up the river channel onto the ice shelf, a clear sign of the mixing between ocean and river water. Understanding these estuaries involves studying supraglacial rivers—rivers that flow on the surface of ice shelves and drain into the ocean. These rivers can have both stabilizing and destabilizing effects. While lakes on ice shelves can increase the risk of collapse, rivers can reduce pressure by carrying water away. However, the new research suggests that estuaries—where rivers meet the ocean—can also be harmful by weakening the ice shelf. The supraglacial river on Petermann began as a typical surface stream but gradually carved a deep channel into the ice. Over time, the river's mouth dropped below sea level, allowing warmer, saltier ocean water to mix with the river, creating the estuary. This mixing process can cause cycles of formation and disappearance as water drains or freezes. The researchers observed that the estuary formed in 2014, then disappeared, and reformed in 2016 and 2018 before a crack in the ice disrupted the river flow. As global temperatures rise, similar estuaries may become more common on ice shelves, increasing the risk of calving events in both Greenland and Antarctica. This research will help scientists monitor and predict future changes in ice shelf stability, crucial for understanding future sea level rise.