Vegetation growth in rivers: influences upon sediment and nutrient dynamics

Hydrological and geomorphological research in river environments has largely ignored the influence of instream vegetation growth; focusing rather on the role of riparian vegetation as a control on bank stability or as a potential buffer for dissolved and particulate material entering the channel from the hillslope. However, in many lowland streams instream vegetation may be abundant and reach high levels of biomass during the growing season. These instream plants (macrophytes) have a significant effect on flow, sediment and nutrient dynamics. Plant growth may cause increased frictional resistance to flow and through flow diversion may have a short- to medium-term influence on instream channel geomorphology. Additionally, this effect of plants upon flow velocities within the channel has an impact on sedimentation patterns. Rooted plants also function as a link between bed sediments and the water column, thus plants have a key role in the cycling of nutrients between these two components of the fluvial system. This, combined with the uptake and temporary storage of nutrients by the plants and the retention of fine sediments within dense plant stands, has the result that plants within rivers are an integral component of nutrient dynamics. A review of research on the role of macrophytes in fluvial system nutrient dynamics is presented and identifies the need for an increased understanding and recognition of the role of plants in the functioning of fluvial systems as a whole.