Biofilms are one of the main biological interphases in river ecosystems, and probably the most important in intermittent rivers, where the importance of the water column is reduced during extended low-activity periods of the hydrological cycle. Biofilms can be understood as microbial consortia of autotrophs and heterotrophs, coexisting in a matrix of hydrated extracellular polymeric substances (EPS). These two main biological components are respectively mainly algae and cyanobacteria on one side, and bacteria and fungi on the other. Micro- and meiofauna also inhabit the biofilm, predating on the organisms and organic particles and contributing to its evolution and dispersal. Biofilms therefore form a highly active biological consortium, ready to use organic and inorganic materials from the water phase, and also ready to use light or chemical energy sources. The EPS immobilize the cells and keep them in close proximity allowing for intense interactions including cell-cell communication and the formation of synergistic consortia. The EPS is able to retain extracellular enzymes and therefore allows the utilization of materials from the environment and the transformation of these materials into dissolved nutrients for the use by algae and bacteria. At the same time, the EPS contributes to protect the cells from desiccation as well from other hazards (e.g., biocides, UV radiation, etc.) from the outer world. On the other hand, the packing and the EPS protection layer limits the diffusion of gases and nutrients, especially for the cells far from the biofilm surface, and this limits their survival and creates strong gradients within the biofilm. Both the biofilm physical structure, and the plasticity of the organisms that live within it, ensure and support their survival in harsh environments or under changing environmental conditions.