Florsheim et al. illustrate how river processes and climate variation increasingly interact with human activity to cause channel incision. Results from their field study in northern California enabled development of a dimensionless metric “relative incision,” to aide in quantifying thresholds of stability in incised alluvial channels. Incision also leads to changes in channel-floodplain hydrologic connectivity. An influx of sediment can serve as an important stratigraphic marker of human activity. For find more example, Stinchcomb et al. studied the distribution of coal alluvium along river valleys of eastern Pennsylvania using an event stratigraphy approach along with specific examples of complex and cascading spatial effects

of human activities. As coal alluvium from mining activities silted up channels, flooding increased, resulting in further distribution of coal alluvium across the floodplains. With over half of the world’s large rivers and virtually all of the rivers in the United States affected by dams (Graf, 2001 and Nilsson et al., 2005), devoting several papers in this issue to investigations of the effects of dams on fluvial forms and processes is appropriate. Yet, each of these papers goes beyond investigating the effects of a single

dam on a river, instead examining the cumulative effects of multiple human interactions over space and time. Skalak et al. studied the Upper Missouri River as a case of the effects of successive dams on fluvial geomorphology, where the downstream effects of one dam are not dissipated before the upstream effects of the next compound screening assay dam occur. The morphology of the reach affected by the interacting dams is distinct from either the typical upstream or downstream effects of singular dams. Skalak and colleagues estimate that 80% of large rivers in the U.S. may have reaches affected

by such interactions. Interacting dams are an example of human manipulations occurring in different places having a cumulative effect on a river or landscape. Freyer and Jefferson consider Oxymatrine the temporal cumulative effects of 150 years of river engineering and dams on the islands and emergent land of the Upper Mississippi River. While eroding islands is the dominant trend in engineered rivers, Freyer and Jefferson examined the patterns and processes of land emergence in a river reach where islands have grown for the last 40 years. They contrast this reach to others where land emergence has not occurred. This analysis of an unusually resilient landscape patch provides one model for guiding restoration designs where unaltered reference conditions no longer exist or where climatic, hydrologic, of geomorphic processes have crossed a threshold and the historical range of variability is no longer applicable. Dammed streams and rivers also provide environmental archives that allow investigation of the geomorphic impacts of land use change in the surrounding watershed. Mann et al.