Swift flow in streams may physically influence the morphology and distribution of plants. I quantified drag as a function of velocity, biomass and their interaction on the trailing canopy of seven European stream species in an experimental flume and evaluated its importance for species distribution. Drag increased at a power of 1.3–1.9 with velocity and 0.59–0.77 with biomass in 75% of the measurements. Velocity and biomass interacted because higher velocity causes reconfiguration and greater internal shelter to unimpeded flow and higher biomass enhances shelter among neighbouring shoots. Increase of drag with velocity did not differ systematically among inherently streamlined or non- streamlined species while increase of drag with biomass was smallest among non-streamlined shoots which provide greater mutual shelter. At low shoot density, inherently streamlined species usually experienced the lowest drag conducive to colonisation and growth in swift flow. At high shoot density, no systematic differences in drag existed between the two morphologies. No clear relationship existed between drag forces, morphology and field distribution of species as a function of current velocity probably because a variety of environmental conditions and plant traits influences distribution. Drag on the trailing canopy usually increased 15- to 35-fold for a 100-fold increase of biomass suggesting that an even distribution of plants at low density across the stream bed offers greater resistance to downstream flow than an uneven distribution with the same biomass confined to dense patches surrounded by open flow channels. Thus, management strategies to ensure a patchy plants distribution should be suitable for combining agricultural drainage and ecological stream quality.
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