Biofilms dominate microbial life in most streams and rivers. Single microbes, initially suspended in the water column, can attach to surfaces where they form microcolonies and ultimately produce an extracellular matrix — the major scaffold of the biofilm. Both structure and function of biofilms are influenced by hydrodynamics, and vice versa, biofilms can shape their hydrodynamic environment. In this talk, I will discuss examples showing how the flow environment may control the transition of single cells to multicellularity. First, I will show how primary cells, depending on the hydrodynamic environment, cluster immediately after their adhesion to a surface — a phenomenon that may be advantageous in harsh environments, such as streams. Next, based on two experimental approaches, I will show how the interplay between hydrodynamics and biofilm surface topography may influence the dispersal of microbial cells and their colonization of native biofilms. Detailed understanding of the biophysical mechanisms underlying an orderly life in streams and rivers is essential to advance our knowledge on microbial biodiversity and ecosystem functioning.