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Sediment Transport and Channel Morphology of Small, Forested Streams1

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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION AUGUST AMERICAN WATER RESOURCES ASSOCIATION 2005 SEDIMENT TRANSPORT AND CHANNEL MORPHOLOGY OF SMALL, FORESTED STREAMS1 Marwan A. Hassan, Michael Church, Thomas E. Lisle, Francesco Brardinoni, Lee Benda, and Gordon E. Grant2 ABSTRACT: This paper reviews sediment transport and channel INTRODUCTION morphology in small, forested streams in the Pacific Northwest region of North America to assess current knowledge of channel stability and morphology relevant to riparian management prac- Forests significantly influence channel morphology tices around small streams. Small channels are defined as ones in and processes in streams. Particular effects are creat- which morphology and hydraulics may be significantly influenced ed by within channel accumulation of large woody by individual clasts or wood materials in the channel. Such chan- debris (LWD) and by channel margin trees. Wood is nels are headwater channels in close proximity to sediment sources, so they reflect a mix of hillslope and channel processes. Sediment critically important in regulating sediment transport inputs are derived directly from adjacent hillslopes and from the and diversifying channel form, thereby also having channel banks. Morphologically significant sediments move mainly major effects on aquatic and riparian ecology (e.g., as bed load, mainly at low intensity, and there is no standard Bisson et al., 1981; Sullivan, 1986; Bilby and Bisson, method for measurement. The larger clastic and woody elements in 1998). This paper presents a review of small forested the channel form persistent structures that trap significant vol- umes of sediment, reducing sediment transport in the short term stream dynamics, drawing primarily from research and substantially increasing channel stability. The presence of such conducted in the Pacific Northwest region of North structures makes modeling of sediment flux in these channels – a America. potential substitute for measurement – difficult. Channel morphol- Wood has its greatest effect in channels with ogy is discussed, with some emphasis on wood related features. The dimensions similar to those of the larger wood pieces, problem of classifying small channels is reviewed, and it is recog- nized that useful classifications are purpose oriented. Reach scale hence smaller streams rather than large rivers are and channel unit scale morphologies are categorized. A “distur- most especially affected. Church (1992) characterized bance cascade” is introduced to focus attention on sediment trans- “small channels” as ones in which individual bed par- fers through the slope channel system and to identify management ticles greatly influence channel morphology. On steep- practices that affect sediment dynamics and consequent channel er gradients, step pool is a dominant bed form. morphology. Gaps in knowledge, errors, and uncertainties have been identified for future research. Absolute width would normally be less than 3 to 5 m. (KEY TERMS: streams; sediment transport; fluvial processes; geo- He characterized “intermediate channels” as ones morphology.) with width much greater than characteristic grain size that might still be influenced by blockage across Hassan, Marwan A., Michael Church, Thomas E. Lisle, Francesco Brardinoni, most or all of the cross-section, most often by fallen Lee Benda, and Gordon E. Grant, 2005. Sediment Transport and Channel Mor- phology of Small, Forested Streams. Journal of the American Water Resources woody debris. In forests, this might include channels Association (JAWRA) 41(4):853-876. up to 20 or 30 m in width. Particle accumulations greatly influence channel morphology, pool riffle being a typical bed form feature. According to the British Columbia channel classification (British Columbia 1Paper No. 04072 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2005). Discussions are open until February 1, 2006. 2Respectively, Assistant Professor (Hassan) and Professor (Church), Department of Geography, University of British Columbia, Vancouver, B.C., Canada V6T 1Z2; Research Hydrologist, USDA Forest Service, Southwest Research Station, Redwood Sciences Laboratory, Arcata, Cali- fornia 95521; Graduate Student, Department of Geography, University of British Columbia, Vancouver, B.C., Canada V6T 1Z2; Research Sci- entist, Earth System Institute, 310 North Mt. Shasta Blvd., Suite 6, Mt. Shasta, California 96067-2230; and USDA Forest Service, Pacific NW Research Station, 3200 SW Jefferson Way, Corvallis, Oregon 97331 (E-Mail/Hassan: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 853 JAWRA HASSAN, CHURCH, LISLE, BRARDINONI, BENDA, AND GRANT Ministry of Forests, 1996a,b), small and intermediate the limited available information on small forested channels have a range of particle size/width ratio streams, certain information accumulated from larger between 0.002 and 1.0, while relative roughness (par- mountain rivers will be included in this paper, and its ticle size/depth) ranges from 0.1 to 1.6. Such channels applicability to small streams will be assessed. are strongly influenced by the entry of wood into First, the fundamental phenomenon of sediment them. In contrast, large channels, with major bar transport is considered under the headings sources, development, meander-bend pools, and crossover rif- mobilization, phases of transport, measurement, vari- fles, are essentially hydraulically controlled, although ability of transport, and modeling. Then channel mor- significant wood accumulations might still occur on phology – the product of sediment transport and bar surfaces. deposition – is introduced under the headings reach Whereas external sediment inputs such as mass morphology, channel units, and channel classification. wasting and bank collapse, along with wood accumu- Finally, slope channel interactions and disturbance lation, tend to dominate channel morphology – hence are considered, with some emphasis on timber har- ecology – in small forest streams, larger channels are vesting and roads. primarily affected by downstream fluvial sediment transport and bank erosion. “Small streams,” as dis- cussed in this paper, would be classified as small or intermediate in the channel typology of Church SEDIMENT TRANSPORT (1992). His definitions are relative because sediment texture and forest morphology influence the size Much research has been conducted into sediment range of such channels in a particular environment. transport processes and sediment transport rate pre- Insofar as they speak to process, they seem more diction in rivers, mostly in relatively large rivers. appropriate than arbitrary classifications by absolute Techniques conventionally used to calculate sediment size. transport in large rivers are not appropriate for head- The focus on small streams stems from recognition water streams where episodic sediment supply from that these represent a distinct class of streams with adjacent slopes, rather than the hydraulic conditions, distinctive morphologies, processes, and dynamics. dominates the sediment transport regime. LWD often There are, of course, small, low gradient channels pre- constitutes a significant portion of material transport- sent in a range of environments, from mountain ed by forested streams (Bilby, 1981) and is a major meadows to lowland tributaries and secondary chan- impediment to the downstream progress of clastic nels. Although important geomorphic and ecologic material (e.g., Megahan, 1982; Rice and Church, 1996; environments in their own right, these channels are Hogan et al., 1998). not considered here. The focus of this paper is the The particulate sediment load of a stream is con- steeper portions of the channel network where episod- ventionally divided into suspended load – particles ic sediment and wood inputs from adjacent slopes moving in the water column with their submerged exert significant control on channel dynamics and weight supported by upwardly directed turbulent cur- morphology. In these channels significant amounts of rents – and bed load – particles moving in contact LWD tend to control the amount of sediment stored with the bed. This division reflects the mechanics of within the channel and impact their stability (e.g., movement and the conventional methods for mea- Swanson et al., 1982c; Bilby and Ward, 1989). surement. A somewhat different division provides The concentration of wood in the channel decreases insight into channel formation and stability. Wash with increasing stream order and channel width, material is relatively fine material that moves direct- implying little impact of wood on channel morphology ly through a reach without being deposited in the of high order streams (higher than fifth order) (e.g., main channel, whereas bed material is the coarser Swanson et al., 1982c; Bilby and Ward, 1989). On the material that is apt to be deposited and form the other hand, wood typically spans the smallest channel bed and banks. Wash material moves in sus- streams and has little hydraulic effect there. Conse- pension and is an important determinant of water quently, attention will be focused on channels that are quality; bed material may move either way. The move- typically of third through fifth orders (as determined ment of bed material along small channels will be the in the field), which are likely to contain significant focus of this part of the review since that is what amounts of wood. determines channel morphology (see Gomi et al., The primary
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