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Effects of Sediment Pulses on Channel Morphology in a Gravel-Bed River

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Effects of Sediment Pulses on Channel Morphology in a Gravel-Bed River Effects of sediment pulses on channel morphology in a gravel-bed river Daniel F. Hoffman† Emmanuel J. Gabet‡ Department of Geosciences, University of Montana, Missoula, Montana 59802, USA ABSTRACT available to it. The delivery of sediment to riv- channel widening, braiding, and fi ning of bed ers and streams in mountain drainage basins material, followed by coarsening, construction Sediment delivery to stream channels in often comes in large, infrequent pulses from of coarse grained terraces, and formation of mountainous basins is strongly episodic, landslides and debris fl ows (Benda and Dunne, new side channels. After the sediment wave had with large pulses of sediment typically 1997; Gabet and Dunne, 2003). This sediment passed, they observed channel incision down to delivered by infrequent landslides and debris supply regime differs from that of channels an immobile bed and bedrock. Cui et al. (2003) fl ows. Identifying the role of large but rare in lowland environments with a more regular conducted fl ume experiments to investigate sed- sediment delivery events in the evolution of sediment supply and is refl ected in the form iment pulses and found that in a channel with channel morphology and fl uvial sediment and textural composition of the channel and alternate bars, the bed relief decreased with the transport is crucial to an understanding of fl oodplain. Processing large pulses of sediment arrival of the downstream edge of the sediment the development of mountain basins. In July can be slow and leave a lasting legacy on the wave, and increased as the upstream edge of the 2001, intense rainfall triggered numerous valley fl oor. Identifying how channels process wave passed. Bartley and Rutherford (2005) debris fl ows in a severely burnt watershed these sediment pulses is critical to an under- investigated channel recovery after sediment in the Sapphire Mountains of Montana. Ten standing of the morphological development of slugs on three Australian rivers and observed a large debris fl ow fans were deposited on the mountainous landscapes. decrease in pool depths for all three, as well as valley fl oor, and investigations focused on the This investigation examines the response of channel widening and bed material fi ning in the channel response to these sediment pulses. a stream channel to a large, sudden increase in Ringarooma River, and bed material coarsening The channel has aggraded immediately sediment supply and presents a conceptual model in Creighton Creek and the Wannon River. upstream of each fan, and braided in reaches of sediment pulse effects on channel morphology Most of the sediment transported through the immediately downstream. Channel incision and sediment transport processes. Intense rainfall fl uvial system is generated through hillslope ero- through the fans has created sets of coarse- in July 2001 triggered 10 debris fl ows that depos- sion in the headwater reaches (Schumm, 1977). grained terraces. The deposition upstream of ited fans of mixed coarse and fi ne sediment in the The rates at which these headwater channels the pulses consists almost exclusively of fi ne channel of Sleeping Child Creek. This provided deliver sediment to downstream reaches affect material, resulting in a median bed material an opportunity to chronicle channel response to the building and modifi cation of alluvial land- size (D50) 1–2 orders of magnitude lower than large sediment pulses soon after the initial distur- forms far downstream of the episodic events the ambient channel material. The volume of bance and to observe how the channel has begun that deliver sediment to the channel. Channel sand being transported is so great that these to process the sediment. depositional processes, such as the construction aggrading reaches can extend hundreds of One of the most obvious effects of a large of bars, fl oodplains, and deltas, depend strongly meters upstream of the fans, with 1–2 m of sediment pulse is a change in channel form. on sediment supply. A large increase in sedi- sand deposited across the entire valley fl oor. Griffi ths (1979) noted channel aggradation, ment supply to channels with established fl ood- Along a 10 km study reach, cross section followed by incision and entrenchment on the plains can lead to fl oodplain aggradation and surveys, longitudinal profi les, and pebble Waimakariri River in New Zealand following terrace construction (Miller and Benda, 2000). counts chronicle channel response to a increased sediment pulses from bank erosion. Understanding how headwater channels process punctuated increase in sediment supply and Beschta (1984) documented channel widen- sediment pulses will help in understanding how provide insight on the processes of sediment ing, followed by subsequent narrowing as a sediment is routed through a channel network, wave dispersal. consequence of increased soil erosion from helping to predict any possible damage to infra- logging activities. Roberts and Church (1986) structure downstream and may assist in predict- Keywords: fl uvial geomorphology, sediment, documented channel incision and fi ning of the ing the results of sediment released from dam debris fl ows, bed load, fi re. bed material in an aggraded channel, following removal projects (Sutherland et al., 2002). increased sediment pulses from timber harvest. Large sediment contributions to stream chan- INTRODUCTION Madej and Ozaki (1996) analyzed changes in nels also affect associated riparian and aquatic channel cross-sectional geometry, and docu- ecosystems. Some riparian fl oodplain plant The morphology of a stream channel is an mented channel aggradation, subsequent deg- species are dependent on the overbank deposi- expression of the supply of water and sediment radation, and channel widening, following tion of fi ne sediments for propagation, whereas increased sediment supply from poor land use deposition of fi ne sediments in salmonid spawn- †E-mail: [email protected]. practices. Following several debris-fl ow sedi- ing areas can signifi cantly reduce spawning ‡E-mail: [email protected]. ment pulses, Miller and Benda (2000) observed success (Carnefi x, 2002). Benda et al. (2003) GSA Bulletin; January/February 2007; v. 119; no. 1/2; p. 116–125; doi: 10.1130/B25982.1; 12 fi gures. 116 For permission to copy, contact [email protected] © 2006 Geological Society of America Effects of sediment pulses on channel morphology in a gravel-bed river found that debris fl ow fans deposited in chan- delivered with debris fl ows. Previous studies Furthermore, the delivery of a large pulse of nels increase the physical heterogeneity of the have documented pool formation associated sediment can affect the sediment transport rate channel. This increase in channel heterogeneity with in-channel LWD (Montgomery et al., of a channel. Cui et al. (2003) found through has implications for riverine ecology, because 1995; Beechie and Sibley, 1997). Benda et fl ume experiments that the introduction of a physical heterogeneity is a vital part of main- al. (2003) found a correlation between LWD sediment pulse signifi cantly reduced the sedi- taining aquatic and riparian biodiversity and and pools in channels with debris-fl ow sedi- ment transport rate upstream of the pulse. This productivity (Benda et al., 2003). ment pulses, where the number of pools was result is in agreement with the observations of In addition, debris fl ows and landslides can proportional to the amount of LWD. The pres- Sutherland et al. (2002), who documented a deliver large amounts of large woody debris ence of pools formed by LWD has implications similar response upstream of a sediment pulse (LWD), and Miller and Benda (2000) docu- for fi sh habitat. Carnefi x (2002) found that bull on the Navarro River in California. Downstream mented channel logjams associated with debris trout (Salvelinus confl uentus) preferentially of a pulse, however, punctuated sediment pulses fl ow deposits. Benda et. al. (2003) found that use pools with LWD cover and documented have been linked to two mechanisms that can up to 80% of the wood in low-order chan- increased spawning recruitment to channels increase sediment transport rates. First, a local nels in Washington’s Olympic Mountains was with these types of habitats. increase in slope at the downstream edge of a sediment pulse increases the tractive force act- ing on the bed and increases sediment transport capacity (Cui et al., 2003; Lisle et al., 1997). Second, Cui et al. (2003) documented that the Montana, USA addition of a pulse of fi ne sediment to a coarse armored channel increased the sediment trans- Study Site port rate and greatly increased the mobility of (46° 6' N, 113° 59' W) the coarse material, often destroying the armored surface layer. Wilcock (1998) described a simi- lar increase in sediment transport rate with the addition of fi ne material to a coarse bed. Study Reach Finally, punctuated sediment delivery often produces pulses or waves, defi ned as transient areas of sediment aggradation in channels, cre- Study Basin ated by large sediment pulses (Lisle et al., 2001). Theoretical, experimental, and fi eld studies have investigated the behavior of sediment waves and the processes responsible for wave translation or A 5 0 5km dispersion. Wavelike behavior of sediment pulses was fi rst described in Gilbert’s (1917) semi- nal paper on sediment waves of placer mining Debris flow fan debris in tributaries of California’s Sacramento Stream gauge and American Rivers. He documented transla- 1 tion of a discrete sediment wave with the apex 2 of the wave moving “like a great body of storm water” in the downstream direction. Numerous 3 studies have documented a similar translational behavior in sediment waves (Griffi ths, 1979; Pickup et al., 1983; Meade, 1985; Turner, 1995; Madej and Ozaki, 1996; Miller and Benda, 4 2000; Kasai et al., 2004a; Bartley and Ruther- ford, 2005). However, other studies of sediment waves in natural rivers and experimental fl umes 5 show a dispersion-dominated behavior (Roberts and Church, 1986; Knighton, 1989; Lisle et al., 6 1997; Dodd, 1998; Lisle et al., 2001).
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