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Quantitative Evaluation of Nivation in the Colorado Front Range

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Quantitative Evaluation of Nivation in the Colorado Front Range Quantitative evaluation of nivation in the Colorado Front Range COLIN E. THORN Department of Geography, University of Maryland, College Park, Maryland 20742 ABSTRACT apparent. The research reported here was intensive and designed to determine which processes are involved in nivation, the magnitude A quantitative evaluation of nivation in a mid-latitude alpine en- of intensification produced by nivation, and the areal and temporal vironment has been derived from an intensive study of two snow distribution of the intensification. The concept of nivation is patches on Niwot Ridge, in the Colorado Front Range. Four re- founded on process intensification; therefore, the collection of search hypotheses were tested: nivation intensifies (1) mechanical comparative data is critical. In this study, data were collected weathering, (2) mechanical transport, (3) chemical weathering, and within snow patches and compared with identical measurements (4) chemical transport. made on nearby snowfree (control) sites. Wherever possible, in- Nivation does not increase the number of freeze-thaw cycles strumentation was deployed so that the data generated were suita- (mechanical weathering); rather, snow patches redistribute the pat- ble for analysis of variance or more specific alternatives, such as tern of occurrence of freeze-thaw cycles by preventing wintertime Duncan's multiple range test. cycles and increasing springtime cycle totals. Intensification of The hypothesis that nivation produces measurable intensification mechanical weathering can only result from increased cycle effec- or acceleration of periglacial weathering and (or) transport proc- tiveness. In contrast to a snowfree site, nivation increases the esses was tested by examination of four component hypotheses — mechanical transport of sand, silt, and clay by an order of mag- namely, nivation produces measurable intensification of (1) nitude. Sheetwash and rill flow dominate mechanical transport. mechanical weathering, (2) mechanical transport, (3) chemical The snowpack itself is protective, sediment removal being focused weathering, and (4) chemical transport. In the strict sense, these downslope of the retreating snow margin. Chemical weathering is subdivisions are arbitrary; in the field, however, they provide man- increased by a factor of two to four by a snow patch. Variations in ageable focal points. weathering rinds indicate that chemical weathering is produced by concentration of meltwater and (or) snowpack free water. RESEARCH AREA AND SITES Within a nivation hollow, chemical and mechanical degradation are approximately equal. On Niwot Ridge, degradation increased The general research area, the Indian Peaks area of the Colorado from 0.0001 mm/yr on a snowfree site to 0.0074 mm/yr within a Front Range, and the individual research sites are shown in Figure nivation hollow. Slope profile through a nivation hollow corre- 1. Selection of a single site with comprehensive characteristics was sponds to slope forms derived theoretically from the continuity impossible; therefore, three sites on the southern flank of Niwot equation. Snow-patch enlargement leads to downslope lengthening Ridge were chosen on the assumption that the well-documented of the nivation hollow, whereas regular, complete meltout pro- higher insolation receipts of southerly aspects (for example, Barry motes incision of the hollow headwall into the hillside. and Chorley, 1970, p. 27—28) would be reflected by a local max- imum in geomorphic activity. The principal sites, Martinelli and INTRODUCTION Longitudinal, were used to study the characteristics of a snow patch on unconsolidated debris and bed rock, respectively. A sub- Nivation is the term introduced by Matthes (1900) to designate sidiary debris site, Saddle, was also studied. the geomorphic impact of late-lying snow patches. The term em- The Martinelli site (Fig. 2) has a single snow patch in winter with braces the idea that such snow patches intensify both weathering a maximum extent of approximately 450 m in length downslope and mass wasting processes. Despite the cursory nature of and maximum width across slope of 175 m. Snow accumulation Matthes's study, the elegance of his paper led to rapid, universal takes place in the lee of a pre-Pleistocene or early Pleistocene acceptance of the nivation concept. A few subsequent papers fo- diamicton of glacial or possibly fluvial origin (Madole, unpub. re- cused on the processes involved in nivation (Lewis, 1936, 1939; sults). Early in the ablation season the snow patch subdivides: a Boch, 1946, 1948; Lyubimov, 1967; Hall, 1975), but the great ma- circular snow patch [Lewis's (1939) classification terminology] oc- jority of investigators described landforms derived from nivation cupies an upper basin approximately 245 m wide and 165 m long processes and gave little or no consideration to the mechanisms in- downslope; a longitudinal snow patch (Lewis, 1939) occupies the volved. The variety of forms assigned a nivation genesis is large, lower basin, which is approximately 83 m wide and 183 m long. ranging from miniature hollows (Nichols, 1963) through fully The two basins are floored by syenite colluvium with little to no formed nivation hollows (St. Onge, 1969) to multistaged cirques vegetation cover. The subsidiary site (Saddle) is 500 m to the east, (Watson, 1966). Overall, the contemporary literature assigns a where a heavily vegetated turf-banked terrace (Benedict, 1970, ter- major role to nivation in the development of periglacial and glacial race 19) is occupied by a transverse snow patch (Lewis, 1939) with landscapes (for example, Flint, 1971, p. 134). Quantitative a maximum winter size of approximately 90 m across slope and 40 verification of nivation, however, is all but absent, a fact highlight- m downslope. The second principal site, the Longitudinal site (Fig. ed by the brief treatment of nivation in such specialized texts as 3), lies 1.4 km west of the other two sites. This small snow patch Bird (1967), Embleton and King (1968, 1975), and Washburn collects against a 4- to 5-m-high wall of gneiss and extends 30 to 40 (1973). In this paper, I examine the mechanisms of nivation as ob- m along the west wall of a narrow gully. served in the Colorado Front Range. Only a small core area of the upper basin at the Martinelli site fails to melt out; this permitted instrumentation throughout the RESEARCH DESIGN snow-patch area. The Martinelli site was the focus for evaluating the intensification of mechanical transport, chemical weathering, Table 1 lists the four established models of nivation; extensive and chemical transport; supporting evidence came from the Saddle studies have characterized all four, and several major conflicts are site. The Longitudinal site was the focus for testing the hypothesis Geological Society of America Bulletin, v. 87, p. 1169-1178, 8 figs., August 1976, Doc. no. 60811. 1169 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1169/3444120/i0016-7606-87-8-1169.pdf by guest on 24 September 2021 1170 C. E. THORN 1 Martinelli Site Green Lakes Valley Area, Colorado Front Range 2 Longitudinal Site (Simplified from U.S.G.S. 7.5 Minute Series) Colorado 3 Saddle Site 0.5 1 Miles Scales -J Map 0 ¿5 1 Kilometers Location Contour Interval 200 Feet (about 61 Meters) Figure 1. Map of the research area showing the three research sites. of intensified mechanical weathering, with supplementary data thaw cycle activity (Table 1). However, many unresolved problems coming from the Martinelli site. concerning freeze-thaw cycles remain, both in the general context of all periglacial environments and in the specific case of nivation. MECHANICAL WEATHERING Older viewpoints (Matthes, 1900, p. 180-181, 189; Lewis, 1936, p. 432; Boch, 1948, p. 1) were that freeze-thaw cycles were fre- The important role assigned to mechanical weathering within ni- quent in the vicinity of snow patches; more recently, the low fre- vation stems from the belief that a snow patch intensifies freeze- quency of freeze-thaw cycles in the Arctic (Cook and Raiche, 1962) and in alpine areas (Fahey, 1973) has been established. Laboratory research has highlighted the complexity of the freez- ing and thawing processes (for example, Williams, 1964). A dis- tinction has been made between a freeze-thaw cycle as a purely thermal event (the oscillation through 0°C) and a geomorphologi- cally significant freeze-thaw cycle that disrupts bed rock. It is not precisely clear what determines the intensity of freeze-thaw weath- ering. All laboratory experiments indicate that the presence of moisture is critical. However, some experiments (for example, Coutard and others, 1970; Potts, 1970) suggest that the frequency of oscillations through the freezing mark is the dominant factor, al- though others (for example, Tricart, 1956; Wiman, 1963) suggest that freezing intensity is at least an important secondary factor. The particular problems of freeze-thaw activity relative to niva- Figure 2. View toward north at Martinelli site (early August 1972); tion are the mechanism and location of intensification. Most work- snow remains in the upper and lower basins, which are separated by the ers have believed that increased freeze-thaw cycle intensity results snowfree dividing ridge. The snowfree control slope is immediately to the from the copious water supply provided by the melting snow patch east (right) of the upper basin. Snow fence is approximately 2 m high. (for example, Matthes, 1900; St. Onge, 1969). A secondary Downloaded
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