When water flows around large rocks in riverbeds, it creates patterns that affect how water moves between the stream surface and spaces beneath the streambed. This underground water movement helps keep streams healthy by naturally cleaning water and providing habitat for organisms.

Most research has studied smooth, artificial channels, but real streams have rough, rocky bottoms made of gravel. We wanted to understand how natural bed roughness affects water flow and underground water exchange around boulders. We also examined what happens when flowing water carves holes around obstacles over time.

Using advanced cameras and computer simulations, we studied water flowing around boulder-like obstacles on three streambed types: smooth surfaces, rough gravel beds, and naturally scoured gravel beds. We measured water speed and pressure at different locations.

Our results show that rough, natural streambeds create much more complex flow patterns than smooth surfaces. Individual gravel pieces generate many small pressure changes throughout the streambed, which may promote more underground water exchange than the boulder alone. When natural scouring occurs, it creates even more complex patterns.

These findings help us better understand how natural streams function and could improve stream restoration projects and predictions of how rivers respond to environmental changes.