ABSTRACT During the past 50 years, geomorphology has become progressively more quantitative, with increasing emphasis on nondimensional metrics that facilitate comparison across field sites, on quantitative conceptual models, on quantification and rigorous dating of geomorphic history, and on exploration of how to quantify physical processes responsible for producing and redistributing sediment. These shifts in emphasis have been facilitated by development of new techniques for collecting and analyzing data, including advances in remote-sensing technology and geochronologic and isotopic methods. During the past half century, the geomorphic community has become more diverse with respect to gender, geographic representation, and disciplinary background, and this has facilitated interdisciplinary approaches to understanding planetary surfaces.

Abstract Dryland channel networks share many similarities with channel networks in more humid regions, but they are also unique in having: extreme temporal and spatial variability in rainfall, runoff, and both hillslope and channel processes; poor integration between tributary and main channels; dominantly ephemeral or intermittent flow; and lack of equilibrium between process and form. Floods are likely to be particularly important in dryland channels, and riparian vegetation exerts a strong influence on channel processes and form. Land managers working in dryland channel networks particularly need to answer the following questions: What is stable? What is the role of disturbance? How do ecosystems depend on physical form and process? This paper explores methods for determining thresholds and resiliency within a channel network and suggests metrics that can be used to assess the condition of a channel segment or entire drainage network relative to management goals. The management metrics focus on flow regime, sediment supply, bed grain size, bedform configuration, width/depth ratio, bed gradient, channel planform, and extent and type of riparian vegetation. For each of these metrics, geological, historical, and systematic records can be used to define the natural range of variability for a particular channel form in the absence of direct land-use impacts. The range of variability present under land use such as military training activities can then be compared to the natural range to assess whether these activities are negatively affecting the dryland channel network. The Yuma Wash drainage in the Yuma Proving Ground, Arizona, is used as a case study.

The South Platte and Republican River basins provide examples of historical channel changes on the western Great Plains. Flow regulation and diversion caused substantial channel narrowing and vegetation encroachment along larger, perennial rivers that head in the Rocky Mountains. Intensive groundwater pumping has reduced the volume and longitudinal connectivity of refuge pools along smaller intermittent or ephemeral channels that head on the plains. A case study from the Pawnee National Grassland of Colorado illustrates the dynamics of intermittent streams, as well as measures that can be taken to protect and restore refuge pools along these streams. The implications of channel change, and the need to protect and rehabilitate rivers, are less widely recognized for smaller rivers of the western Great Plains than for the larger, perennial rivers. Our objectives in this chapter are to provide a regional context for understanding changes in smaller plains rivers during the past century by reviewing the diversity of channel types and historical changes in the western Great Plains, and to briefly explore the dynamics of smaller plains rivers and the challenges to preserving these riverscapes.

Abstract Human activities impact an estimated 98% of rivers in the United States. This chapter summarizes impacts associated with pioneer societies, commercial activities, and public works. In pioneer societies, individuals or small groups undertake activities such as timber harvest, agriculture, or navigation improvements. Nineteenth-century placer mining of gold along rivers of California's Sierra Nevada is used as a case study. Commercial activities are conducted by groups of people seeking profit through industry, commerce, or agriculture. Commercial activities impacted rivers much more extensively than most activities of pioneer societies. Impacts to water quality, and particularly the U.S. Geological Survey National Water Quality Assessment (NAWQA) program, provide a case study. Results from the 1991–1995 NAWQA program indicate that approximately half the sites sampled in urban areas have surface-water contamination exceeding levels at which adverse biological effects can occur in aquatic biota. Public works such as dam and levee construction undertaken by local and federal governmental agencies caused massive alteration of river systems. Channelization is used as a case study of the impacts of public works on rivers; more than 56,000 km of waterways were channelized by the Corps of Engineers and the Soil Conservation Service after 1940. The net effect of human activities in the United States has been to disconnect rivers from adjacent hillslopes, floodplains, and valley bottoms, underlying hyporheic and groundwater zones, and from headwaters and downstream processes. Because a connected river is a functioning ecosystem, rather than simply a canal for moving water and sediment, disconnection simplifies and impoverishes the ecosystem. This has resulted in widespread loss of biological diversity in rivers of the United States.