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Valley Formation by Fluvial and Glacial Erosion

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Valley Formation by Fluvial and Glacial Erosion Valley formation by ¯uvial and glacial erosion David R. Montgomery Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA ABSTRACT Cross-valley pro®les from the west slope of the Olympic Peninsula, Washington, are used to investigate the relative effects of ¯uvial and glacial erosion on valley formation. Unlike most ranges where glaciers and rivers sequentially occupied the same valleys, neighboring valleys in the Olympic Mountains developed in similar lithologies but were subject to different degrees of glaciation, allowing comparison of the net effect of glacial and ¯uvial processes integrated over many glacial cycles. Upslope drainage area was used to normalize comparisons of valley width, ridge-crest-to-valley-bottom relief, and valley cross-sectional area as measures of net differences in the mass of rock excavated from below ridgelines for 131 valley-spanning transects. Valley width, relief, and cross-sectional areas are similar for glaciated, partly glaciated, and unglaciated (¯uvial) valleys with drainage areas of ,10 km2, but diverge downslope. Glaciated valleys draining .50 km2 reach two to four times the cross-sectional area and have to 500 m greater relief than comparable ¯uvial valleys; partly glaciated valleys have intermediate dimensions. At dis- tances of .5 km from valley heads, the cumulative upstream volume of rock removed to form valleys is two to four times greater in glacially incised valleys than in ¯uvial valleys. The ®nding of strong differences in the net result of valley excavation by ¯uvial and glacial erosion supports the interpretation that alpine glaciers are more effective erosional agents than are rivers and implies that large alpine valleys deepened and enlarged in response to Pleistocene glaciation. Keywords: glacial, ¯uvial, erosion, valley formation. INTRODUCTION et al., 1996), recent studies in areas of rapid (Augustinus, 1992b), but there has been no There is no longer serious debate about uplift document river incision rates compara- direct quantitative comparison of valley mor- whether glaciers can erode their beds and ble to high erosion rates in glaciated regions phology in comparable glaciated and ungla- sculpt topography, but debate continues over (e.g., Burbank et al., 1996). Moreover, sedi- ciated basins. Here I report evidence for sub- whether glaciers are more erosive than rivers. ment yield data are dif®cult to translate di- stantial differences in the geomorphic Some workers ®nd evidence for greater rates rectly into erosion rates owing to long-term expression of ¯uvial and glacial erosion in the of glacial erosion than ¯uvial erosion (e.g., sediment storage in ¯uvial systems (Dunne et Olympic Mountains, Washington. Clague, 1986; Braun, 1989; Harbor and War- al., 1998; Goodbred and Keuhl, 1998) and the burton, 1992, 1993; Clayton, 1996; Hallet et potential for lag times between erosion and STUDY AREA al., 1996; Kirkbride and Mathews, 1997), sediment delivery to exceed the duration of The Olympic Mountains of western Wash- whereas others ®nd evidence for low rates of typical glacial cycles (Church and Slaymaker, ington comprise an accretionary wedge that glacial erosion or little difference between ¯u- 1989). rose from the Paci®c Ocean in the late Mio- vial and glacial erosion rates (e.g., Sugden, Of central importance to understanding cene in response to convergence of the Juan 1976, 1978; LindstroÈm, 1988; Hicks et al., feedback among climate change, erosion, and de Fuca plate and North America (Tabor and 1990; Summer®eld and Kirkbride, 1992; Heb- tectonics in alpine areas is whether glaciation Cady, 1978). Rock-uplift rates across the don et al., 1997; Lidmar-BergstroÈm, 1997). leads to more erosion and greater relief than range have been steady since ca. 14 Ma, and However, comparisons of ¯uvial and glacial can be attributed to ¯uvial processes. Whipple the range is thought to have been in topo- erosion rates face the fundamental problem et al. (1999) analyzed river longitudinal pro- graphic steady state for most of that time that glaciers and rivers sequentially occupied ®les and argued that any in¯uence of higher (Brandon et al., 1998). The Olympic Moun- the same valleys, making the signatures of erosion rates in glacial periods is likely to tains are just south of the maximum extent of glacial and ¯uvial processes on valley size dif- have only limited impact on relief for equilib- Pleistocene ice sheets but were the site of re- ®cult to deconvolve (Roberts and Rood, rium river pro®les in mountain systems where peated episodes of alpine glaciation. Geologic 1984). Global comparisons are further com- development of threshold bedrock slopes pre- mapping of the western Olympics shows that plicated by the fact that data come from wide- vents attainment of steeper valley sides some valleys in the range repeatedly hosted ly different settings, such as cold-based con- (Schmidt and Montgomery, 1995). Physically large Pleistocene valley glaciers, whereas oth- tinental ice sheets, warm-based alpine based models have been proposed for the de- ers had either no glaciers or smaller glaciers glaciers, and temperate, tropical, and arid riv- velopment of U-shaped glacial valley forms generally restricted to their headwaters (Wash- ers in a broad range of tectonic contexts. Al- from initially V-shaped ¯uvial valleys (Harbor ington Division of Geology and Earth Re- though the most comprehensive comparison et al., 1988; Hirano and Aniya, 1988; Harbor, sources Staff, 2001). The west side of the indicates that sediment yields are higher in 1992), and the in¯uence of rock strength on Olympic Mountains presents an unusual op- glaciated than in unglaciated regions (Hallet glacial valley form also has been investigated portunity in that a series of neighboring val- q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; November 2002; v. 30; no. 11; p. 1047±1050; 4 ®gures. 1047 Figure 1. Shaded relief map of west slope of Olympic Mountains, Wash- ington, showing locations of basins studied, cross sections (white bars), and moraines (black are- as) as portrayed on 1: 100 000 scale geologic maps (Washington Divi- sion of Geology and Earth Resources Staff, 2001). leys with similar geology had strikingly dif- were limited to drainage areas of 0.1 km2 to ferent glacial histories but shared the same 500 km2, because ®ner scale features are not general climate variability through time. As well resolved and glaciers did not extend the range has been in a topographic steady much beyond this range in valley size. state since the late Miocene, a well-developed Comparisons of valley morphometry were ¯uvial valley network must have evolved prior normalized by the drainage area upslope of to onset of Pleistocene glaciation, which mod- each cross section. Mean slope and precipita- i®ed an initial ¯uvially incised valley network. tion also were calculated for the drainage ba- River systems exhibit systematic scaling in sin upslope of each cross section. Valleys that which channel and valley size increase with head highest in the range developed large val- drainage area or river discharge (Leopold and ley glaciers in the glacial climate, but the Maddock, 1953), and fjord size similarly dominantly ¯uvial valleys on the western scales with drainage area or ice discharge slope of the Olympic Mountains also extend (Roberts and Rood, 1984; Augustinus, 1992a). to high elevations where precipitation reaches Such relationships suggest that systematic dif- 4±5 m´yr21. Modern drainage divides do not ferences in the morphometry of valleys on the necessarily coincide with Pleistocene ice di- western slope of the Olympic Peninsula be- vides, but the two should be similar in the tween those formed primarily by ¯uvial ero- high-relief valleys in the core of the range, and sion and those sequentially incised by both any glacial spillover would account for a pro- glacial and ¯uvial erosion can be used to gressively smaller proportion of the drainage gauge long-term morphologic effects of gla- area downstream through the valley network. ciation on valley form. The relief given by the difference between the present maximum and minimum elevations on METHODS each valley-spanning topographic cross sec- The spatial distribution of glacial moraines tion provides only a minimum constraint on was used to assess the relative degree of gla- the local bedrock relief, because postglacial cial in¯uence on nine valleys (Fig. 1). Fully alluvial valley ®lls now occupy valley ¯oors. glaciated valleys (Hoh, South Fork Hoh, and The methodology I employ does not evaluate Figure 2. Comparison of valley morphometry Queets) had large, valley-spanning Fraser and erosion rates directly, but allows assessment vs. drainage area for (A) valley width; (B) ridge-crest-to-valley-bottom relief; and (C) pre-Fraser age moraines. Partially glaciated of the integrated long-term signature of ero- valley cross-sectional area. Black squares valleys (Bogachiel, Sams, and Tshletshy) had sional processes on the volume of material re- represent valleys with major alpine glaciers less extensive glacial in¯uences with small moved from between valley walls. Hence the (Hoh, South Fork Hoh, and Queets), trian- Fraser age moraines located partway down the approach allows examination of the relative ef- gles represent partly glaciated valleys (Bo- gachiel, Tshletshy, and Sams), and circles valley. Unglaciated valleys had no mapped ®cacy of ¯uvial and glacial
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