The article discusses how water flows between a river and the ground below over time. The researchers studied the Methow River in Washington to understand how groundwater moves into the river. They found that the length of groundwater seepage changes with the seasons and can affect the river's ecosystem. The study used mathematical models to predict these changes and showed that the groundwater seepage can vary by several kilometers. This information is important for understanding how rivers and aquifers interact and how it can impact the environment, like the life stages of chinook salmon.