Abstract

A quantitative evaluation of nivation in a mid-latitude alpine environment has been derived from an intensive study of two snow patches on Niwot Ridge, in the Colorado Front Range. Four research hypotheses were tested: nivation intensifies (1) mechanical weathering, (2) mechanical transport, (3) chemical weathering, and (4) chemical transport.

Nivation does not increase the number of freeze-thaw cycles (mechanical weathering); rather, snow patches redistribute the pattern of occurrence of freeze-thaw cycles by preventing wintertime cycles and increasing springtime cycle totals. Intensification of mechanical weathering can only result from increased cycle effectiveness. In contrast to a snowfree site, nivation increases the mechanical transport of sand, silt, and clay by an order of magnitude. Sheetwash and rill flow dominate mechanical transport. The snowpack itself is protective, sediment removal being focused downslope of the retreating snow margin. Chemical weathering is increased by a factor of two to four by a snow patch. Variations in weathering rinds indicate that chemical weathering is produced by concentration of meltwater and (or) snowpack free water.

Within a nivation hollow, chemical and mechanical degradation are approximately equal. On Niwot Ridge, degradation increased from 0.0001 mm/yr on a snowfree site to 0.0074 mm/yr within a nivation hollow. Slope profile through a nivation hollow corresponds to slope forms derived theoretically from the continuity equation. Snow-patch enlargement leads to downslope lengthening of the nivation hollow, whereas regular, complete meltout promotes incision of the hollow headwall into the hillside.

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