Review on Biogeomorphology in Rivers: Processes and Scales M.J. Baptist Delft University of Technology, the Netherlands WL | Delft Hydraulics, the Netherlands DRAFT version. ABSTRACT: This literature review discusses the influence of biogeomorphological processes at different spatio- temporal scales for fluvial systems. It uses a hierarchical classification to describe these processes. 1. INTRODUCTION The management of rivers in the Netherlands changes from fixing and stabilising the river system to letting go the natural processes. The aim is to restore the former dynamic conditions, in order to get a sustainable and more diverse river system. One of the challenges of river restoration is to redesign the floodplains in such a way that it will synergistically strengthen the functions of safety against flooding and natural diversity. The main problem here is that the hydraulic roughness of vegetation in combination with sedimentation in floodplains decreases the required discharge capacity. Therefore, a new management approach is proposed in which rejuvenation of landscape elements will clear the way for discharge of water and simultaneously will maintain the systems diversity. This new approach requires understanding of the interaction between the biotic and abiotic components of river systems. This review therefore provides a literature review on the interaction between organisms and geomorphological factors, i.e. biogeomorphology. This review emphasises on processes and scales and describes biogeomorphological processes for different scales. 2. TEMPORAL AND SPATIAL SCALES FOR RIVER SYSTEMS Geomorphological processes occur for time scales ranging from microseconds, which are relevant for turbulence velocities, up to hundreds millions of years for geological processes. The spatial scales are similarly wide, from millimetres for capillary flows in sediments up to the continental and global scales. Scale refers in this review to the spatial or temporal dimension of an object or process (Turner et al., 1989). Kirkby (1990) presents an example for the wide variety in scales for river systems, see Figure 1. One way to deal with the spatial and temporal scales of patterns and processes of fluvial systems is to describe these in terms of a hierarchy of scales. Roughly speaking, spatial and temporal scale levels are coupled, in that small-scale phenomena are associated with small-scale processes, and large-scale phenomena with large-scale processes, or with small-scale processes that are coherent over a large scale (De Vriend, 2000). A hierarchical classification for linked processes at multiple scales is a common method to cope with scale linkage problems of landscapes (Klijn, 1997). For river basins these hierarchical classifications have also been made. Naiman et al. (1992) and Townsend (1996) give an overview of various hierarchical approaches for stream and catchment classification. One of the most useful approaches is the classification of Frissell et al. (1986). They present a framework for hierarchical classification of streams and stream habitats. In this framework, streams and their watershed environments are classified within the context of geomorphic features and events, and spatio- temporal boundaries are identified. Frissell et al. distinguish between the stream system, segment system, reach system, pool/riffle system and the microhabitat system, see Figure 2.