Changes in Channel Morphology Over Human Time Scales
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32 Changes in Channel Morphology Over Human Time Scales John M. Buffington 32.1 INTRODUCTION Montgomery, 1999a; Nelson et al., 2009) that, in turn, affect the availability of riparian and aquatic habitats Rivers are exposed to changing environmental condi- (Lisle, 2005; Burke et al., 2006; May et al., 2009; Moir tions over multiple spatial and temporal scales, with the et al., 2009). Sediment routing models have been devel- imposed environmental conditions and response poten- oped for examining a wide range of responses, including tial of the river modulated to varying degrees by human textural adjustment, reach-scale changes in sediment activity and our exploitation of natural resources. Wa- storage and channel slope, changes in channel width and tershed features that control river morphology include planform, effects of sediment pulses, and landscape topography (valley slope and channel confinement), evolution (Hoey and Ferguson, 1994; Nicholas discharge (magnitude, frequency, and duration of runoff et al., 1995; Benda and Dunne, 1997a, 1997b; Cui and events), sediment supply (volume, calibre and frequency Parker, 2005; Coulthard et al., 2008; Parker et al., 2011; of sediment delivery), and vegetation (riparian commu- Mosselman, Chapter 9, this volume). In addition, our nities (bank strength, roughness) and in-channel wood knowledge of the form and function of rivers has ex- debris). River stability and response to changing envi- tended beyond traditional studies of sand- and gravel-bed ronmental conditions are highly dependent on local rivers to include a wide variety of channel types and context (channel type and associated degrees of freedom; geomorphic settings; in particular, a tremendous amount the nature of the imposed sediment, hydrologic, and of work has been done on step-pool channels (Comiti and vegetation regimes; imposed anthropogenic constraints; Mao, Chapter 26, this volume). Numerous recent inves- and the legacy of past natural and anthropogenic tigations also document the role of vegetation in affect- disturbances). ing channel morphology and response over multiple Understanding the processes responsible for channel spatial and temporal scales, including controls on chan- change and assessing river stability has long been of nel width, grain size, bedforms, roughness, sediment interest to geologists and engineers (Gilbert, 1877; du transport, alluviation, and rates of landscape lowering Boys, 1879; Davis, 1889; Lindley, 1919; Shields, 1936; (see review by Montgomery et al., 2003). Recent studies Lane, 1937; Mackin, 1948; Leopold and Maddock, 1953; also document the role of animals in modulating geo- Schumm, 1969). Over the last several decades, interest in morphic processes and channel characteristics (Rice this topic has grown exponentially as a result of envi- et al., Chapter 19, this volume). Furthermore, historical ronmental legislation that has spurred greater interdis- studies demonstrate the startling extent of human dis- ciplinary collaboration amongst physical and biological turbance to rivers worldwide (Schumm, 1977; Collins scientists studying riverine ecosystems and watershed et al., 2003; Surian and Rinaldi, 2003; Nilsson processes. During this time, considerable progress has et al., 2005; Chin, 2006; Surian, 2006; Walter and been made in understanding channel response and in Merritts, 2008), fueling the notion that an anthropogenic elucidating biophysical interactions. epoch has emerged (Crutzen, 2002; Meybeck, 2003). For example, progress has been made in understanding This chapter reviews channel change over human time textural and structural response of gravel streambeds and scales (10–1–102 a), which encompasses small-scale consequent effects on bed mobility (Dietrich et al., 1989; adjustments resulting from seasonal changes in water- Church et al., 1998; Wilcock, 1998; Buffington and Gravel-bed Rivers: Processes, Tools, Environments, First Edition. Edited by Michael Church, Pascale M. Biron and Andre´ G. Roy. Ó 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd. 436 JOHN M. BUFFINGTON shed inputs to large-scale changes in reach morphology resulting from infrequent floods or decadal to centennial changes in climate (wet/dry cycles). The chapter begins with a review of scales of channel change and the spatial and temporal variability of channel response. This is followed by an examination of the available approaches for predicting channel change which, despite recent advances in the field, are still mainly limited to lowland rivers and assumptions of equilibrium conditions. Tran- sient channel responses are explicitly accounted for in numerical models, but temporal variability of channel Figure 32.1 Spatial and temporal scales of channel condition is typically absent in other quantitative models response variables in alluvial rivers: grain size (individ- of channel response. Quantifying temporal variability is ual grains to textural patches); width, depth (cross-sec- critical for assessing channel condition, planning resto- tion to subreach variation in average channel geometry); ration design, and predicting effects of climate change on bedforms (micro-forms (e.g., ripples, bedload sheets) to riverine ecosystems. It is hypothesized that hydrocli- channel units (e.g., bar, pool, step, riffle)); and stream mate, and its control on both the shape of the hydrograph gradient (reach-scale channel aggradation/incision or and the relative size of floods, is a first-order control on change in sinuosity). Partly after Knighton (1998). the natural variability of channel morphology. Lewin, 1978; Bisson et al., 1982; Church and 32.2 SCALES OF CHANNEL CHANGE Jones, 1982; Wood-Smith and Buffington, 1996; Halwas and Church, 2002; Hassan et al., 2008). Rivers can exhibit a broad range of responses to changing (3) Altered channel geometry (changes in local cross- inputs of water, sediment, and vegetation over human sectional width, depth, and downstream variation time scales. Channel response may range from small- of those features; e.g., Trimble, 1997). scale adjustment of channel characteristics (grain size, (4) Altered stream gradient due to reach-scale aggra- width, depth) to large-scale alteration of reach morphol- dation/incision and changes in channel sinuosity. ogy and planform pattern. Here, stream gradient is distinguished from valley gradient, which is not adjustable in the short term. 32.2.1 Changes in Channel Characteristics Larger scales of channel response reflect the cumula- Successive, overlapping, spatial and temporal scales of tive action of smaller-scale processes, particularly sed- morphologic response in alluvial channels include: iment transport of bed and bank materials. Hence, a progression of successive scales of response can be (1) Grain-scale adjustment, comprising: envisioned, with grain-size adjustment being the first- (a) local changes in grain size, packing, protru- order response (Figure 32.1). Furthermore, because sion, and friction angle (Fenton and Ab- alluvial rivers exhibit mutually adjusting channel char- bott, 1977; Church, 1978; Buffington acteristics, changes in any one parameter can influence et al., 1992; Johnston et al., 1998); all of the others. (b) development of micro-grain forms (e.g., par- The extent of channel change that occurs for a given ticle clusters, stone cells; Laronne and Car- disturbance depends on the amount of work accom- son, 1976; Brayshaw, 1984; Church plished by the event (flood magnitude times duration; et al., 1998; Strom and Papanicolaou, 2008); Wolman and Miller, 1960) and the time needed for a (c) formation of textural patches (i.e., grain-size given scale of response to occur. Wolman and Miller’s facies; Dietrich et al., 1989; 2006; Rice, 1994; (1960) classic magnitude–frequency argument empha- Paola and Seal, 1995; Buffington and sizes the effectiveness of frequent, moderate-sized Montgomery, 1999a; Laronne et al., 2001; events in accomplishing geomorphic work over the long Nelson et al., 2009; Yuill et al., 2010). term, but large-scale changes in morphology require (2) Changes in the type, size, and frequency of bed large events (Figure 32.2). Similarly, different temporal topography, ranging from micro-bed forms (e.g., scales of disturbances (seasonal to centennial) will ex- ripples, bedload sheets; Whiting et al., 1988) to hibit characteristic scales of response. Seasonal changes macro-bed forms or channel units (individual bar, are frequent, typically small-magnitude events that will pool, step, and riffle topography; Jackson, 1975; lead to similarly small degrees of channel change (e.g., CHANGES IN CHANNEL MORPHOLOGY OVER HUMAN TIME SCALES 437 show that steep, confined channels in central Idaho recover from massive debris-flow inputs within 5–10 years because of high transport capacities and no oppor- tunities for floodplain storage of sediment, while recovery from sediment waves may take decades in lower-gradient, unconfined channels (Megahan et al., 1980; Madej and Ozaki, 1996). Over longer time scales (decades to cen- turies), wet/dry climate cycles may lead to corresponding cycles of river incision/aggradation (Rumsby and Figure 32.2 Work and scale of channel change accom- Macklin, 1994), expansion/contraction of channel width plished for flood events of different magnitude and (Schumm and Lichty, 1963), and changes in the com- duration (after Costa and O’Connor, 1995). Total work plexity of bed topography and textural patches. Overall, (shaded area) is identical for the events, but the scale of the