Effects of animals on the transport of solids (e.g., sediments) can modify earth-surface processes and landforms (role as geomorphologic agents) and resources for other organisms (role as ecosystem engineers). Therefor, we studied the impact of crayfish (Orconectes linosus) behavior on sand-gravel erosion and bottom habitat over riffle-pool sequences in experimental streams.We manipulated the avaibility of crayfish food and reugia. Refugia availbility had clear effects on overall crayfish activity. The establishmentof dominance hierarchies among the crayfish produced patterns in caryfish activity that differed between riffles and pools. Crayfish activity significantly affected sand and gravel erosion. This direct erosion caused by crayfish should be lower than that caused by floods. Crayfish distinctly increase bedform roughness (riffles), decreased sand dune height (pools), shifted the transition between gravels (riffles) and sand (pools) downstream, decreased sand in gravel interstices (riffles), decreased filamentous algae growth on gravels (riffles), and decreased biofilm cover on sand dunes (pools). These sediment changes due to crayfish activity at baseflow could largely affect population and community structure of the benthos, as well as egg survival of gravel-breeding fish.