How do groundwater and surface streams exchange water?

In hydrologic sciences, the exchange of water between groundwater and surface streams is governed by the relative elevation of the water table compared to the water level in a stream or lake. This interaction is driven by pressure gradients.1

• Groundwater to Stream: When the groundwater table is higher than the water level in a stream, a pressure head develops that forces groundwater to flow into the stream channel. This process provides the base flow that sustains stream levels between precipitation events.1
• Stream to Groundwater: Conversely, if the water table is lower than the level of the stream, the pressure gradient induces water to flow from the stream into the surrounding groundwater system.1

On a global scale, groundwater contributes approximately 30 percent of the water found in total river runoff. This communication occurs directly wherever a stream or lake intersects the water table. These exchanges are fundamental components of the hydrologic cycle, reflecting the continuous movement and redistribution of water within the Earth-atmosphere system.12