Impact of the flow velocity and sedimentation on microbial biofilms in the stream Ilm (Thuringia Germany) Streams are linear definite one dimensional structures and several theoretical concepts aiming the stream ecology are existing. The River Continuum Concept (RCC) for instance describes streams as a continuous series of physical gradients and associated biotic adjustments indicating that the stream organisms interact with the longitudinal changing environment (Vannote et al. 1980). The products of streams are highly appropriate to be transported downstream than reaching the river bottom (Schönborn 2003). Most of the organic matter available in streams is of allochthonious origin as leaves from the riparian zones (99%). After entering the stream allochtonious material undergoes three phases of processing: leaching microbial colonization (conditioning) and fragmentation by physical forces and invertebrate feeding (Fisher and Likens 1973). The amount of terrestric organic carbon transported or transformed by stream and river ecosystems world wide is about 2 Pg per year whereas the metabolic capacity in these fluvial systems may result from microbial attachements as biofilms (Battin et al. 2008).