Introduction to the special issue on discontinuity of fluvial systems

Available online 10 April 2013 Fluvial systems include natural and human-created barriers that modify local base level; as such, these discontinuities alter the longitudinal flux of water and sediment by storing, releasing, or changing the flow path of those materials. Even in the absence of distinct barriers, fluvial systems are typically discontinuous and patchy. The size of fluvial discontinuities ranges across scales from 10 m, such as riffles, to 10 m, such as lava dams or major landslides. The frequency of occurrence appears to be inversely related to size, with creation and failure of the small features, such as beaver dams, occurring on a time scale of 10 to 10 years and a frequency of occurrence at scales as low as 10 m. In contrast, larger scale discontinuities, such as lava dams, can last for time scales up to 10 years and have a frequency of occurrence of approximately 10 m. The heterogeneity generated by features is an essential part of river networks and should be considered as part of river management. Therefore, we suggest that “natural” dams are a useful analog for human dams when evaluating options for river restoration. This collection of papers on the studies of natural dams includes bedrock barriers, log jams and beaver dams. The collection also addresses the discontinuity generated by a floodplain — in the absence of an obvious barrier in the channel — and tools for evaluation of riverbed heterogeneity. It is completed with a study of impact of human dams on floodplain sedimentation. These papers will help geomorphologists and river managers understand the factors that control river heterogeneity across scales and around the world. © 2013 Elsevier B.V. All rights reserved. River networks are typically patchy and discontinuous systems (Montgomery, 1999), where patches are formed by the fluvial or “extrafluvial” (sensu Ely et al., 2012) processes that generate fluvial discontinuities. These discontinuities are often distinct barriers to water and sediment transport that modifies local base level, which has long been recognized as an important component of the fluvial system (Mackin, 1948). Fluvial discontinuities alter the longitudinal flux of water and sediment by storing, releasing, or changing the flow path of those materials. They can disrupt the progression of a river toward a graded system (sensu Gilbert, 1877) by delaying river incision at time scales up to 10 to 10 years (Ely et al., 2012). Conversely, the catastrophic floods that may accompany the failure of fluvial barriers generate rapid incision that is far in excess of the flooding that would be generated by meteorological conditions alone (Butler and Malanson, 2005; O'Connor and Beebee, 2009); these are recognized as flood hazards critical to river management (Costa and Schuster, 1988). The effects of natural dams are sufficient to control valley formation, where the scale of impact varies according to the scale of the barrier (e.g., Korup et al., 2010; Kramer et al., 2012).