Two massive rock glaciers in Alaska's Wrangell–St. Elias National Park are moving down mountain valleys at different speeds. One is moving more than 5 feet (1.5 meters) per year, while the other is moving less than half that pace. These glaciers are only a few miles apart and formed under similar environmental conditions. A team led by researchers from Southern Methodist University (SMU) has discovered that the difference in their movement is influenced by factors like the slope's direction and the presence of water. The study, published in the Journal of Geophysical Research: Earth Surface, used satellite radar data collected over five years to map seven active rock glaciers in the park between 2018 and 2022. Rock glaciers are slow-moving landforms composed of rocky debris held together by ice. They are considered important indicators of permafrost health, as they reflect long-term changes in the frozen ground. Qingyu Sui, the study's lead author and a Ph.D. candidate at SMU, noted that although air temperatures peak once a year, the rock glaciers studied showed two distinct periods of increased movement. This suggests that factors other than air temperature, such as snowmelt and rainfall, are driving their motion. The researchers found that seasonal changes in the rock glaciers are influenced by snowmelt in spring and late-summer rains, which provide the necessary water to lubricate the layers beneath the surface. Solar radiation also plays a key role, as it determines whether the subsurface is warm enough for this water to reach the layers where movement occurs. At Sourdough Peak, the fastest-moving glacier, the first speed-up occurred 72–84 days after spring snowmelt began, and the second occurred after late-summer rains. In contrast, the nearby McCarthy Creek rock glacier showed almost no seasonal speed-up. The difference in movement between Sourdough Peak and McCarthy Creek was traced to sunlight exposure. Sourdough Peak faces south and is in an open area, receiving more solar energy, which warms the subsurface and allows meltwater to reach the layers where motion happens. McCarthy Creek, on the other hand, is partially shaded by the valley wall, keeping its upper layers frozen and acting as a barrier to movement. Zhong Lu, a professor at SMU and corresponding author on the study, emphasized that satellite radar technology allows for precise measurements of surface movement in remote areas. When combined with hydrologic and thermal modeling, these measurements can reveal how climate conditions interact with local factors like slope, direction, and solar exposure. The findings have implications beyond Alaska. Rock glaciers store water that mountain communities rely on, especially during dry years. Most rock glaciers have never been measured directly, but the SMU team's method offers a way to monitor thousands of them globally without needing to physically access the ice. This approach could improve understanding of water availability and climate change impacts in high mountain regions.