Geologic and physiographic controls on bed-material yield, transport, and channel morphology for alluvial and bedrock rivers, western Oregon Jim E. O’Connor1,†, Joseph F. Mangano1,2, Scott W. Anderson1,3, J. Rose Wallick1, Krista L. Jones1, and Mackenzie K. Keith1 1U.S. Geological Survey, Oregon Water Science Center, 2130 SW 5th Avenue, Portland, Oregon 97201, USA 2Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523, USA 3Department of Geography, University of Colorado–Boulder, Boulder, Colorado 80309, USA ABSTRACT geometry and sediment supply. At the scale (Stanford and Ward, 1993; Yarnell et al., 2006). of western Oregon, the physiographic and Most fundamentally, the distinction relates to the The rivers of western Oregon have di- lithologic controls on the balance between balance between bed-material supply and river verse forms and characteristics, with channel bed-material supply and transport capacity transport capacity (Gilbert, 1877, 1914; Howard, substrates ranging from continuous alluvial exert far-reaching infl uence on the distribu- 1980; Whipple, 2004). Rivers in which the long- gravel to bare bedrock. Analysis of several tion of alluvial and nonalluvial channels and term channel transport capacity exceeds bed- measurable morphologic attributes of 24 val- their consequently distinctive morphologies material supply (termed supply- or detachment- ley reaches on 17 rivers provides a basis for and behaviors—differences germane for un- limited rivers) will typically fl ow over bedrock comparing nonalluvial and alluvial channels. derstanding river response to tectonics and beds for part or much of their courses. Where Key differences are that alluvial reaches have environmental perturbations, as well as for supply meets or exceeds transport capacity greater bar area, greater migration rates, and implementing effective restoration and moni- (transport-limited rivers), channel beds are typi- show systematic correlation among variables toring strategies. cally formed of a continuous mantle of alluvial- relating grain size to bed-material transport bed material. capacity. We relate these differences between INTRODUCTION This categorization, however, masks sub- channel types to bed-material transport rates stantial complexity. As summarized by Church as derived from a coupled regional analysis of The rivers of western Oregon have channel (2002, 2006) and Lisle (2012), the morphology empirical sediment yield measurements and beds ranging from fully alluvial to bedrock. A and transport conditions of alluvial channels physical experiments of clast attrition dur- local history of in-stream gravel mining in con- involve interrelations among fl ow, channel and ing transport. This sediment supply analy sis junction with an ongoing permitting process valley characteristics, sediment supply, and sedi- shows that overall bed-material transport for continued mining have prompted a series of ment grain size. These interrelations commonly rates for western Oregon are chiefl y con- investigations of bed-material production, trans- create conditions of bed-material fl ux, channel trolled by (1) lithology and basin slope, which port, and channel morphology across this spec- form, bed elevation, and bed-sediment textures are the key factors for bed-material supply trum of channel types in western Oregon (Wal- such that the bed material entering the system is into the stream network, and (2) lithologic lick et al., 2010, 2011; Jones et al., 2011, 2012a, balanced, at decadal to millennial time scales, control of bed-material attrition from in- 2012b, 2012c). These studies, expanded upon by that exiting, i.e., the graded river of Mackin transport abrasion and disintegration. This and synthesized here, show the importance of (1948). This system, classically depicted by the bed-material comminution strongly affects (1) geologic and physiographic controls on bed- Lane-Borland balance between stream energy bed-material transport in the study area, re- material production and in-stream gravel fl ux; and sediment fl ux (Lane, 1955), has been subject ducing transport rates by 50%–90% along and (2) the differences between fully alluvial to more than a century of scrutiny because of the the length of the larger rivers in the study channels and those that locally fl ow on bedrock many pragmatic implications of predicting allu- area. A comparison of the bed-material in terms of predicting transport rates, bed-mate- vial channel behavior and morphology in con- transport estimates with the morphologic rial characteristics, and channel morphology. sequence of changing environmental conditions. analyses shows that alluvial gravel-bed chan- Channels with bedrock beds and margins nels have systematic and bounding relations Alluvial, Bedrock, and have also been studied extensively, but chiefl y between bed-material transport rate and at- Mixed-Bed Channels for their broad role in pacing valley incision and tributes such as bar area and local transport landscape evolution (summarized by Turowski, capacity. By contrast, few such relations are The distinction between alluvial and bedrock 2012). Finer-scale studies have mostly focused evident for nonalluvial rivers with bedrock or channels has broad implications regarding long- on bedrock channel forms (summarized by mixed-bed substrates, which are apparently term channel incision (Howard, 1980; Whipple, Wohl, 1998; Whipple, 2004; Richardson and more infl uenced by local controls on channel 2004; Turowski et al., 2008a, 2008b; Turowski, Carling, 2005), erosional processes (Whipple 2012), channel morphology (Montgomery et al., 2000; Wohl and Merritt, 2001; Johnson and et al., 1996; Montgomery and Buffi ngton, 1997; Whipple, 2007; Goode and Wohl, 2010a), and †E-mail: [email protected] Tinkler and Wohl, 1998), and physical habitat transport conditions (Goode and Wohl, 2010b; GSA Bulletin; March/April 2014; v. 126; no. 3/4; p. 377–397; doi:10.1130/B30831.1; 11 fi gures; 4 tables; Data Repository item 2014049; Published online 7 January 2014. For permission to copy, contact [email protected] 377 © 2014 Geological Society of America O’Connor et al. Hodge et al., 2011). Few “bedrock” channels, The regional geology is important to our ents less than 0.005 and would be considered however, have continuous bedrock beds; most analysis, and we have aggregated existing map- transport or response reaches in the Montgom- have patches, locally extensive, of alluvium in ping into six main lithologic groupings (Fig. 1). ery and Buffi ngton (1997) categorization. Study and fl anking the channel, leading to the terms The Paleozoic and Mesozoic rocks of the tec- reaches were defi ned on the basis of broad-scale “mixed-bed” or “semi-alluvial” channels (How- tonically accreted Klamath terrane underlie geomorphic characteristics, with boundaries ard, 1980, 1998; Lisle, 2012; Turowski, 2012). much of the southwestern part of the study area. typically corresponding to major confl uences, The degree of alluvial cover has been hypoth- Uplifted Tertiary marine sediments constitute changes in valley confi nement, and extent of esized to modulate bedrock erosion (Gilbert, the Coast Range sedimentary province underly- tidal infl uence. The 24 separate reaches summa- 1877; Sklar and Dietrich, 2001; Finnegan et al., ing much of the western part of the study area. rized here do not include the tidal reaches iden- 2007; Turowski et al., 2007). Only recently, Tertiary marine volcanic rocks within the Coast tifi ed in these previous studies. The short fl uvial however, have studies focused on the alluvial Range and the Columbia River Basalt Group reaches of Hunter Creek and the Chetco, Wil- characteristics of channels that locally or con- in the northern part of the study area have been son, and Miami Rivers defi ned in our previous tinuously fl ow on bedrock (Chatanantavet and grouped together into what we term the Coast studies (Wallick et al., 2010; Jones et al., 2011, Parker, 2008; Goode and Wohl, 2010b; Hodge Range volcanics province. The axis of the Cas- 2012c) have been aggregated in this assess- et al., 2011). cade Range is underlain by Quaternary volcanic ment so that each one of these rivers is repre- In this study, we directly compare alluvial rocks of the High Cascades province, and these sented by single valley reaches. The resulting and bedrock channels of western Oregon, and rocks are fl anked to the east and west by Tertiary 24 study reaches range from <5 km to as long we relate their distribution and character to volcanic rocks grouped into the Western Cas- as 115.7 km (Table 1). The Umpqua and Rogue basic controls on bed-material fl ux and trans- cades lithologic province. Basins, broad valleys, River basins have the most reaches, refl ecting port capacity. These differences and distinc- and coastal plains are underlain by Quaternary distinct morphological differences along these tions have implications for understanding river sediment. large and long rivers and their major tributaries, response to tectonics and environmental pertur- Our analysis is based on morphologic obser- while several of the shorter coastal rivers each bations, as well as for implementing effective vations from 24 valley reaches within the 17 consist of a single reach. All of the study reaches restoration and monitoring strategies. rivers . Analyzed reaches span the spectrum of have gravel bed material. fully alluvial to bedrock (Fig. 2). Our measure- Western Oregon Study Area and ments of channel and bar area, channel migration Valley Reach Classifi