The study looked at how water flows between a river and the ground near Vienna. They used a computer model to simulate the movement of water in the ground and river. By studying a gravel bar in the river, they found that water can move in and out of the ground depending on the river's water level. When the river is low, water goes from the ground into the river. But when the river rises, water moves from the river into the ground. This back-and-forth movement affects how water mixes between the river and the ground near the gravel bar.