Geological Society of America Bulletin, published online on 8 January 2016 as doi:10.1130/B31340.1 Bedrock incision and strath terrace formation in a forested catchment Rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment, Cascade Range, Washington Brian D. Collins1,†, David R. Montgomery1, Sarah A. Schanz1, and Isaac J. Larsen2 1Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195-1310, USA 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003-9297, USA ABSTRACT strath abandonment corresponds with histor- pension), macro-abrasion (fracturing of the bed- ical anthropogenic removal of fluvial wood, rock into pluckable or entrainable sizes through Measurements of channel bed and bank suggesting that the relative abundance of flu- the collision with particles in transport), pluck- incision into bedrock coupled with mapping vial wood may influence episodes of vertical ing (hydraulic removal of blocks), dissolution, and radiocarbon dating of strath terraces in bedrock incision by affecting the retention of and cavitation (Whipple et al., 2000). The domi- the West Fork Teanaway River, Washington, alluvium on streambeds. nance and efficacy of an erosional process in a provide insight into rates and mechanisms natural channel depend in part on lithology, joint of river incision and strath terrace forma- INTRODUCTION spacing, fractures, and bedding planes (Whipple tion in a forested landscape. The West Fork et al., 2000). Physical and chemical weather- drains 102 km2 of the slowly exhuming Bedrock incision by rivers drives the topo- ing can greatly enhance erosional processes southeastern North Cascade Range, and it graphic evolution of mountain landscapes and (Howard, 1998; Whipple et al., 2000; Stock is rapidly eroding its bed and creating strath can leave a morphologic signature in the form of et al., 2005; Hancock et al., 2011; Han et al., terraces in its lower reach. Minimum vertical strath terraces, which provide important records 2014; Small et al., 2015) by increasing rough- incision, measured annually relative to nails for interpreting tectonic and climatic history ness (Hancock et al., 1998; Huda and Small, embedded in the streambed, was greater in the (Pazzaglia, 2013). Because of bedrock incision’s 2014) and decreasing rock strength and thereby seasonally exposed, weathering-dominated, importance, it has been the subject of a number of enhancing susceptibility to abrasion, expanding high-flow channel (mean = 10.9 mm yr–1) theoretical and experimental (see review by Lamb fractures along which blocks are removed by than in the perennially wet, abrasion-domi- et al., 2015) and field studies (e.g., Hancock et al., plucking (Hancock et al., 1998; Whipple et al., nated, low-flow channel (3.8 mm yr –1), docu- 1998; Tinkler and Parish, 1998; Whipple et al., 2000), and comminuting rock into smaller frag- menting unsteady lowering of the channel 2000; Hartshorn et al., 2002; Stock et al., 2005; ments (Stock et al., 2005). margin. Ages of radiocarbon-dated materials Johnson et al., 2010; Lamb and Fonstad, 2010; The efficacy of at least some erosional and from alluvium on strath terraces, 0.1 m to Cook et al., 2013; Inoue et al., 2014). However, weathering processes is also influenced by the 5.4 m above the water surface, suggest three few studies have linked incisional processes with thickness of alluvial cover. For example, labora- episodes of strath abandonment at maximum lateral planation of straths or their subsequent tory experiments predict maximum abrasion ages of ca. A.D. 830, A.D. 1560, and A.D. 1890, incision and stranding as strath terraces (e.g., of streambeds having partial bedrock exposure and average incision rates of 1.3 mm yr–1, Montgomery, 2004; Finnegan and Balco, 2013; associated with intermediate sediment supplies 1.4 mm yr –1, and 7.4 mm yr –1 for the oldest to Johnson and Finnegan, 2015). Here, in a field that provide the “tools” for abrading the bed youngest surfaces, respectively. Weathering- study of a river that is rapidly eroding its bed without protecting the bed from abrasion (Sklar promoted vertical incision in the high-flow and creating strath terraces, we measured annual and Dietrich, 2001). While most applications of channel provides a mechanism for “top- vertical and lateral incision to address theoretical this concept to interpreting river incision his- down” rapid lateral strath planation in which and laboratory predictions on the relative roles tories have focused on regional- or basin-scale scour of alluvium on incipient strath terraces of erosion and weathering processes in shaping influences on stream power or the supply and incorporates the surface into the high-flow channels (e.g., Hancock et al., 2011; Small et al., caliber of sediment (e.g., changes to base level, channel, allowing rapid removal of bedrock 2015). We then coupled these measurements climate, or tectonic uplift rates), reach-scale weathered during wetting and drying cycles. with incision rates determined from strath ter- processes that control the routing or retention Relationships among channel width, channel races using radiocarbon dating, and we propose of sediment in channels, such as landslides that confinement by bedrock terrace risers, mod- a mechanism of rapid strath planation and how form temporary dams in rivers, could control eled bankfull shear stress, and alluvial bed it may, by adjusting channel morphology, inter- rates of bedrock incision by altering the dis- cover suggest that rapid channel widening nally limit incision. tribution of alluvial and bedrock reaches (e.g., could also internally limit vertical incision Ouimet et al., 2007). In forested regions, such by slowing incision as shear stresses decline Background a mechanism could also be provided by in- and more alluvium is retained on the bed. channel wood accumulations, which can trans- The timing of the most recent (ca. A.D. 1890) Bedrock channels erode by processes that form channels from bedrock to alluvial by alter- include abrasion (removal of rock by impact ing the balance between sediment transport and †[email protected] from saltating particles in the bed load or in sus- supply (Montgomery et al., 1996). GSA Bulletin; Month/Month 2015; v. 1xx; no. X/X; p. 1–18; doi: 10.1130/B31340.1; 15 figures; 6 tables.; published online XX Month 2015. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 © 2016 Geological Society of America Geological Society of America Bulletin, published online on 8 January 2016 as doi:10.1130/B31340.1 Collins et al. Cross-channel variation in processes and than, 2002; Finnegan et al., 2007; Spotila et al., et al., 2014). However, Gallen et al. (2015) rates of bedrock incision shapes channel cross- 2015) or susceptibility to weathering (Hancock recently proposed this effect could, in many riv- sectional and planform geometry, which in turn et al., 2011; Johnson and Finnegan, 2015), and ers, be the result of a systematic bias introduced controls hydraulics and the distribution of ero- thus the efficacy of lateral strath planation of by use of the modern streambed as the datum for sive power across the channel (for review, see bedrock channels (Montgomery, 2004; García, measuring terrace heights. Hancock et al., 2011). Differential incision rates 2006; Wohl, 2008). Channel planform may in a channel’s low-flow and high-flow bed (in also influence rates and patterns of lateral ero- Approach this paper, we use “low-flow channel” inter- sion; Finnegan and Balco (2013) suggested that changeably with “channel thalweg” and “high- a braided channel is more likely to accomplish To investigate how different erosional pro- flow channel” interchangeably with “channel lateral planation because the channel is more cesses operate in space and time to vertically margins”) can result from differences in weath- prone to incise laterally along both margins, and horizontally incise rivers and create straths, ering (Stock et al., 2005; Hancock et al., 2011; rather than just one margin. we undertook a field study of a rapidly incis- Small et al., 2015) or from whether cover or Incision and terrace creation have long been ing river in a region with slow rock uplift and tool effects dominate (Turowski et al., 2008). recognized as inherently unsteady (Gilbert, no known active faulting, the forested West Differences in rock erodibility can also be a 1877; Mackin, 1937; Bull, 1990) due to tempo- Fork Teanaway River in the southeastern North primary influence on the shape of river longi- ral variation in uplift (e.g., Pazzaglia and Gard- Cascades of Washington State (Fig. 1). Previ- tudinal profiles (Duvall et al., 2004; Allen et al., ner, 1993; Schoenbohm et al., 2004), base-level ously, we reported 1 yr average bedrock inci- 2013). Locally rapid incision is often associ- fall, and knickpoints that propagate upstream sion from the study site (Stock et al., 2005). ated with knickpoints that propagate upstream (e.g., Harvey and Wells, 1987; García et al., Here, we report direct field measurements of or that initiate in steeper, smaller-drainage- 2004, Bishop et al., 2005), or climate-forced vertical bed incision and lateral bank reces- area parts of a channel network (Crosby and variations in the relative supply of water and sion measured relative to nails in the riverbed Whipple, 2006). However, several field studies sediment (e.g., Pazzaglia and