Editorial Introduction to the special issue on discontinuity of fluvial systems Denise Burchsted a, ⁎, Melinda Daniels b , Ellen E. Wohl c a Keene State College, 229 Main St. Keene, New Hampshire 03435 b Stroud Water Research Center, 970 Spencer Road Avondale, PA 19311 c Dept. of Geosciences, Colorado State University, Ft. Collins, CO 80523, United States abstract article info Available online 10 April 2013 Fluvial systems include natural and human-created barriers that modify local base level; as such, these dis- continuities 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 0 m, such as riffles, to 10 4 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 0 to 10 1 years and a frequency of occurrence at scales as low as 10 1 m. In contrast, larger scale discontinuities, such as lava dams, can last for time scales up to 10 5 years and have a frequency of occurrence of approxi- mately 10 4 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 sedimenta- tion. 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 4 to 10 5 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). Even without the presence of distinct barriers, the patchiness of river networks is best described as a discontinuum (Poole, 2002). By generating storage and release of water, discontinuities increase the complexity of flow paths across and along the river corridor, modify- ing temperature and biogeochemical regimes. These effects generate longitudinal and lateral heterogeneities of instream and riparian habitats across river networks (Burchsted et al., 2010). Given the im- portance of discontinuity on river form and function, it should be assessed as part of river management (Brierley et al., 2002; Snyder et al., 2009); however, river restoration often relies on the dominant ecological paradigm of the River Continuum Concept (Vannote et al., 1980) when evaluating human impacts on rivers (e.g., Hart and Poff, 2002). This special issue further examines the impact of specific fluvial discontinuities on river form and process and also presents tools for evaluation of discontinuity and heterogeneity. Here, we start by categorizing discontinuities as those generated by living, dead, and non-living material and agents. We refer to the ones generated by non-living materials and agents as abiotic discon- tinuities. These span a wide range of spatial and temporal scales. At the smallest scale, they include features such as riffles and they are typically generated by autogenic fluvial processes. Examples of abiotic discontinuities at the largest scale include major ice and sediment dams, lava dams, and landslides, all of which tend to be extrafluvial. In contrast, biotic discontinuities are formed by living Geomorphology 205 (2014) 1–4 ⁎ Corresponding author. Tel.: +1 603 535 3179. E-mail address: dburchsted@plymouth.edu (D. Burchsted). 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.04.004 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph