8 Review of effects of large floods in resistant-boundary channels

Abstract

Gravel-bed channels formed in coarse sediment or mixed bedrock and coarse sediment have higher thresholds of resistance to fluvial erosion than channels formed in finer-grained sediment. These higher erosional thresholds are likely to be exceeded only during large floods. Variables that influence the geomorphic role of floods in resistant-boundary channels include the flood-generating mechanism (hydroclimatic and damburst processes), position within a drainage basin, erosional threshold, sediment supply, land use, in-channel wood, riparian vegetation, and time since last flood. Geomorphic effects of floods include transport of sediment and wood, alteration of channel and valley morphology, and channel incision. Ecological effects of floods include alteration of physical and chemical characteristics of the river and floodplain, and changes in riparian and aquatic community composition and structure. Channel restoration and management must account for the occurrence and effects of infrequent, large magnitude floods by focusing on river process rather than river form. The net result of downstream trends in numerous physical variables is that the aggregate population of large floods creates the greatest geomorphic effects in headwater channels, whereas individual large floods are likely to be most geomorphically important in the middle portion of drainage basins.