1
Introduction

Rivers are the shapers of terrestrial landscapes. Very few points on Earth above sea level do not lie within a drainage basin. Even points distant from the nearest channel are likely to be influenced by that channel. Tectonic uplift raises rock thousands of meters above sea level. Precipitation falling on the uplifted terrain concentrates into channels that carry sediment downward to the oceans and influence the steepness of adjacent hill slopes by governing the rate at which the landscape incises. Rivers migrate laterally across lowlands, creating a complex topography of terraces, floodplain wetlands, and channels. Subtle differences in elevation, grain size, and soil moisture across this topography control the movement of ground water and the distribution of plants and animals.

Investigators have begun to quantify the extent to which rivers influence the surrounding landscape. Stream ecologists ask, "How wide is a stream?" and address the question by using isotopic signatures to analyze food web data indicating exchanges of matterand energy between aquatic and terrestrial biotic communities (Muehlbauer et al. 2014). Geomorphologists ask, "How large is a river?" and address the question by defining signatures - emergent properties of sets of processes acting on a river landscape - and envelopes - the dynamic penetration of a signature across the landscape (Gurnell et al. 2016b). In each case, the answer is, "Wider and larger than surface appearances might suggest."

Throughout human history, people have settled disproportionately along rivers, relying on them for water supply, transport, fertile agricultural soils, waste disposal, and food from aquatic and riparian organisms. People have also devoted a tremendous amount of time and energy to altering river process and form. We are not unique in this respect: ecologists refer to various organisms, from beaver to some species of riparian trees, as ecosystem engineers in recognition of their ability to alter their environment. Humans are unique, however, in the extent to and intensity with which we alter rivers. In many cases, river engineering has unintended consequences, and effectively mitigating these consequences requires that we understand rivers in the broadest sense, as shapers and integrators of landscape.

Geomorphologist Luna Leopold once described rivers as the gutters down which flow the ruins of continents (Leopold et al. 1964). His father, Aldo Leopold, described the functioning of an ecosystem as a "round river," to emphasize the cycling of nutrients and energy. Rivers can be thought of as having a strong unidirectional and linear movement of water, sediment, and other materials. Rivers can also be thought of as more broadly connected systems with bidirectional fluxes of energy and matter between the channels of the river network and the greater environment. This volume emphasizes the latter viewpoint.

Rivers are not simply channels. Various phrases have been used to describe the integrated system of channels, floodplain, and underlying hyporheic zone, including "the river system," "the fluvial system," "the river ecosystem," and "the river corridor." Regardless of the exact words used, the intent is to recognize that the active channel is integrally connected to adjacent surface and subsurface areas by fluxes of material and organisms. The three legs of the tripod of physical inputs that support a river corridor are inputs of water, sediment, and large wood from adjacent uplands. Although large wood has received less attention than water and sediment inputs, the historical abundance of large wood in regions with forested uplands or floodplains, along with observations of the geomorphic effects of large wood in the few remaining natural river corridors, indicates that large wood significantly influences river process and form. The material inputs of water, sediment, and wood are redistributed within the river corridor, stored for varying lengths of time, and eventually transported to the ocean, to another long-term depositional environment (e.g. alluvial fan or delta), or - for water - back to the atmosphere or ground water.

Each of the primary inputs to a river corridor can be described in terms of natural regimes that occur in the absence of human alterations in land cover, river form, flow regulation, and the water table, and in terms of altered regimes associated with human activities. The natural flow regime can be characterized with respect to magnitude, frequency, duration, timing, and rate of rise and fall of water discharge (Poff et al. 1997). Human alterations of the flow regime can be quantified using indicators of hydrologic alteration (Richter et al. 1996; Poff et al. 2010). The natural sediment regime can be characterized with respect to inputs, outputs, and storage of sediment (Wohl et al. 2015b). Because records of sediment flux analogous to those of gaged stream discharge do not exist, human alterations of the sediment regime can be inferred from the occurrence of sustained changes in river process and form that result from altered sediment dynamics. The natural wood regime can be characterized with respect to magnitude, frequency, duration, timing, rate, and mode of wood recruitment, transport, and storage within river corridors (Wohl et al., 2019). As with sediment, insufficient systematic records exist of wood flux in the absence of human influences to quantify changes in the natural wood regime, but the effect of human influences can be inferred from sustained changes in river process and form (e.g. Collins et al. 2012).

The details of how materials from uplands enter a river corridor and move through it are partly governed by the spatial context of the corridor (Figure 1.1). Context here includes valley geometry (downstream gradient, valley-bottom width relative to active channel width), position in the network, base-level stability, and substrate erosional resistance (Wohl 2018a). Valley geometry influences the energy available for changes in river form and the space available to accommodate change. Steep river reaches typically correspond to relatively narrow valleys and coarser sediment or bedrock (Livers and Wohl 2015). Lower-gradient reaches are more likely to have wide valley bottoms relative to channel width, as well as floodplains or secondary channels. Position in the network can influence the sensitivity of a river corridor to fluctuations in relative base level: commonly, the lower portions of a river network are more likely to incise in response to relative base-level fall or aggrade in response to relative base-level rise. Base-level stability influences river corridor configuration in that a river reach may be incising or aggrading irrespective of inputs of water, sediment, and large wood because of base-level instability (e.g. Schumm 1993). Substrate erosional resistance describes the ability of the channel and floodplain substrate to resist erosional changes. Resistance derives from substrate composition (grain size, stratigraphy, bedrock lithology; e.g. Finnegan et al. 2005) and from the presence of riparian vegetation (e.g. Gurnell 2014).

Figure 1.1 Schematic illustration of the primary inputs to river corridors (water, sediment, large wood) and the context in which they interact with one another and with the river form to create the integrative river corridor characteristics listed in the lower portion of the figure. (See color plate section for color representation of this figure).

Human activities can modify inputs and context. Although people typically do not alter the actual valley geometry, they do commonly alter the effective valley geometry by building levees, regulating flow and reducing flood peaks, or stabilizing the banks, each of which limits the interactions between channel and floodplain. Analogously, construction of grade controls or dams affects local base-level stability, and land drainage or bank stabilization modifies substrate erosional resistance.

Interactions between inputs and valley context create the characteristics of the river corridor listed in the lower row of Figure 1.1: spatial heterogeneity, nonlinear behavior, connectivity, resilience, and integrity. Connectivity and nonlinear behavior are introduced in this first chapter. The other concepts are covered in subsequent ones.

1.1 Connectivity and Inequality

Contemporary research and conceptual models of river form and process increasingly explicitly recognize the important of connectivity. Connectivity, sometimes referred to as coupling (e.g. Brunsden and Thornes 1979), is multifaceted. Hydrologic connectivity can refer to the movement of water down a hillslope in the surface or subsurface, from hillslopes into channels, or along a channel network (Pringle 2001; Bracken and Croke 2007). River connectivity refers to water-mediated fluxes within the channel network (Ward 1997). Sediment connectivity can refer to the movement, or storage, of sediment down hillslopes, into channels, or along channel networks (Harvey 1997; Fryirs et al. 2007a,b; Kuo and Brierley 2013; Bracken et al. 2015). Biological connectivity refers to the ability of organisms or plant propagules to disperse between suitable habitats or between isolated populations for breeding. Landscape connectivity can refer to the movement of...