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2002 Hancock.Pdf Downloaded from gsabulletin.gsapubs.org on December 21, 2010 Geological Society of America Bulletin Numerical modeling of fluvial strath-terrace formation in response to oscillating climate Gregory S. Hancock and Robert S. Anderson Geological Society of America Bulletin 2002;114;1131-1142 doi: 10.1130/0016-7606(2002)114<1131:NMOFST>2.0.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes Geological Society of America Downloaded from gsabulletin.gsapubs.org on December 21, 2010 Numerical modeling of ¯uvial strath-terrace formation in response to oscillating climate Gregory S. Hancock* Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA Robert S. Anderson Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA ABSTRACT many thousands of years. The transition stream-transport capacity, vertical erosion from valley widening to downcutting and rates decrease, making lateral planation rela- Many river systems in western North terrace creation occurs in response to sub- tively more important. Flood plains are the re- America retain a ¯uvial strath-terrace rec- tle input changes affecting local divergence sult of this lateral planation, and strath ter- ord of discontinuous downcutting into bed- of sediment-transport capacity. Formation races are vestiges of these ¯ood plains, rock through the Quaternary. Their impor- of terraces lags by several thousand years formed as river downcutting leads to ¯ood- tance lies in their use to interpret climatic the input changes that cause their plain abandonment (Gilbert, 1877). Mackin events in the headwaters and to determine formation. (1937) proposed that incipient terraces form long-term incision rates. Terrace formation Our results suggest that use of terrace during periods of river stabilityÐi.e., grad- has been ascribed to changes in sediment ages to set limits on the timing of a speci®c ed conditionsÐwhen lateral planation of val- supply and/or water discharge produced by event must be done with the knowledge that ley walls produces wide valley ¯oors. Graded late Quaternary climatic ¯uctuations. We the system can take thousands of years to conditions occur when a river can transport all use a one-dimensional channel-evolution respond to a perturbation. The incision rate of the imposed sediment load and is achieved model to explore whether temporal varia- calculated in the ®eld from the lowest ter- through a balance between sediment load and tions in sediment and water discharge can race in these systems will likely be higher size, discharge, and channel slope (Mackin, generate terrace sequences. The model in- than the rate calculated by using older ter- 1937). According to Mackin (1937), the width cludes sediment transport, vertical bedrock races, because the most recent ¯uvial re- of the valley ¯oor is related to the duration of erosion limited by alluvial cover, and lat- sponse in the ®eld is commonly downcut- the graded period. Both Gilbert (1877) and eral valley-wall erosion. We set limits on ting associated with declining sediment Mackin (1937) suggested that the beveling of our modeling by using data collected from input since the Last Glacial Maximum. the bedrock strath and the deposition of the the terraced Wind River basin. Two types This apparent increase in incision rates is alluvial mantle occurred simultaneously and of experiments were performed: constant- observed in many river systems and should that the channel geometry remained stable period sinusoidal input histories and variable- not necessarily be interpreted as a response while the channel migrated across the valley period inputs scaled by the marine d18O to an increase in rock-uplift rate. ¯oor. The beveled strath and thin alluvial cov- record. Our simulations indicate that er distinguish strath from ®ll terraces, which strath-terrace formation requires input Keywords: ¯uvial features, landscape evo- have no underlying bedrock platform and can variability that produces a changing ratio lution, modeling, river terraces, sediment be many tens of meters thick. of vertical to lateral erosion rates. Straths supply. These hypotheses suggest that strath for- are cut when the channel ¯oor is protect- mation requires alternation between ``graded'' ed from erosion by sediment and are INTRODUCTION periods of extensive lateral planation, and abandonedÐand terraces formedÐwhen ``nongraded'' periods of vertical incision. Riv- incision can resume following sediment- Fluvial strath-terrace sequences record river ers may obtain graded conditions when avail- cover thinning. High sediment supply pro- incision through time and the timing of envi- able stream power equals the critical power, motes wide valley ¯oors that are abandoned ronmental perturbations to ¯uvial systems. de®ned as the stream power necessary to as sediment supply decreases. In contrast, Fluvial strath terraces are abandoned river transport enough sediment to maintain grade wide valleys are promoted by low effective ¯ood plains with thin mantles of alluvium (Bull, 1979, 1990). Under these conditions in- water discharge and are abandoned as dis- overlying a beveled bedrock platform (Fig. 1). cision rates (i.e., rates of vertical erosion) are charge increases. Widening of the valley Conceptually, strath-terrace formation requires at a minimum, allowing extensive valley wid- ¯oors that become terraces occurs over changes in the ratio of vertical incision to lat- ening by lateral planation. Strath terraces are eral planation in a downcutting stream. Gilbert created when the available stream power be- *E-mail: [email protected]. (1877) postulated that as sediment load nears comes greater than this critical power, leading GSA Bulletin; September 2002; v. 114; no. 9; p. 1131±1142; 11 ®gures. For permission to copy, contact [email protected] q 2002 Geological Society of America 1131 Downloaded from gsabulletin.gsapubs.org on December 21, 2010 HANCOCK and ANDERSON et al., 1991; Reheis et al., 1991; Repka et al., 1997). The cause for this acceleration is not clear, but could indicate regional intensi®ca- tion of climate, base-level, and/or tectonic change. Alternatively, Mills (2000) has pro- posed that apparent late Cenozoic acceleration of incision rates deduced from many terrace sequences in the southeastern United States is an artifact of averaging over longer periods as terrace age increases. In our modeling, we at- tempt to test this hypothesis. The timing, duration, and mechanisms of strath-terrace formation are dif®cult to infer solely from ®eld observations because terrace sequences represent incomplete records, are Figure 1. Photograph looking downstream along the Wind River below Dubois, Wyoming. dif®cult to date, and formed during ¯uvial The width of the view represents ;1 km at the center. Above the active ¯ood plain, three conditions that differ from the present. Here terrace levels (ages: WR-1, ca. 20 ka; WR-3, ca. 120 ka; and WR-7, ca. 600 ka) mapped we explore strath-terrace formation in re- by Chadwick et al. (1997) can be seen along the course of the river. sponse to climate oscillation by using a nu- merical stream-pro®le simulation. This ap- proach differs signi®cantly from previous modeling efforts focused on the generation of to incision and ¯ood-plain abandonment 1993; Leopold and Wolman, 1957; Schumm ¯uvial terraces (e.g., Boll et al., 1988; Veld- (Bull, 1979). In this conceptual model, switch- and Khan, 1972; Smith and Smith, 1984). kamp, 1992; Veldkamp and Vermeulen, 1988) ing between these two states can occur by al- On the basis of this evidence, valley wid- because we rely on physically based rules and tering the available stream power (e.g., chang- ening and strath formation are proposed to oc- a realistic set of stream inputs. First, we dis- ing discharge or slope), and/or the critical cur during periods of maximum glaciation cuss the model framework and rule set, the power (e.g., sediment load or size; Bull, (e.g., Baker, 1974; Bull, 1991; Howard, 1970; constraints on initial and boundary conditions, 1979). Available stream power may be in- Ritter, 1967; Sinnock, 1981), although glaciers and our modeling strategy. Second, we discuss creased, and hence, terraces may be created by within the headwaters of a river system are not the results and their implications for terrace incision, through basin-scale changes such as required (e.g., Pierce and Scott, 1982). Valley formation and interpretation. Speci®cally, we base-level fall, hydrologic changes induced by widening and strath formation are initiated by explore the following questions: (1) Why do climate change, and/or increased tectonic ac- increased sediment supply and hydrologic terraces and terrace sequences form? (2) How tivity (e.g., Merritts et al., 1994; Pazzaglia et changes associated with a larger and later- is terrace formation related in time to forcing al., 1998).
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