Riverbeds are not just simple structures - they have complex flow patterns that affect how water moves between rivers and underground. By studying the Aa River in Belgium, researchers found that the traditional way of estimating water exchange between rivers and aquifers may not be accurate. They compared two methods: a simple 1D calculation and a more detailed 3D computer model. The 3D model showed that water flow is not just up and down but also sideways, especially near the riverbanks. This means that the traditional method may underestimate how much water is moving between the river and the ground. More research is needed to figure out when the simple method is good enough and when we need to use more advanced models.