NIVATION IN WASHINGTON’S PALOUSE HILLS

Nivation includes geologic processes associated with snow and its meltwater: erosion, mass wasting, physical and chemical weathering. These processes lead to a depression which, through positive feedback, results in a larger snow bank and a larger nivation hollow. Kirkham et al. (1931) and Rockie (1934) proposed that the hollows of the Palouse Hills were caused by nivation associated with snowdrifts. Google Earth images reveal 83 depressions in the USGS 7.5-minute Tucannon Quadrangle. The hollows’ mean dimensions are: width, 124 m; length, 109 m; depth, 19 m.

96% of the hollows, which have a mean elevation of 637 m, lie in the southern half of the quadrangle. The south gets more snow and keeps it longer because the south has elevations of up to 2400 m whereas the north has a maximum elevation of only 1700 m. The prevalence of fluvial over periglacial processes is likely in the north, where the Tucannon River system has incised the landscape approximately 200 m; the valley of the Touchet River system in the south is only 75 m deep.

A dominant northeastern aspect of the nivation hollows was expected because of drifting snow associated with the prevailing southwesterly winds and less sunlight on north-facing slopes. However, the nivation hollows in the Tucannon Quadrangle have a dominant northwestern aspect, together with abundant north-facing hollows. Valley heads of first-order streams in the quadrangle have a dominant southwestern aspect. Daily photographs of one basin showed snow persisting about four days longer than on the surrounding area. We confirm that the hollows of the Palouse Hills are indeed nivation basins.