Snowdrift Erosion in the Palouse

American Geographical Society

Snowdrift Erosion in the PalouseAuthor(s): W. A. RockieSource: Geographical Review, Vol. 41, No. 3 (Jul., 1951), pp. 457-463Published by: American Geographical SocietyStable URL: http://www.jstor.org/stable/210963 .

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SNOWDRIFT EROSION IN THE PALOUSE

W. A. ROCKIE

*:. c C A N A D A THE Palouse region of south-

eastern Washington and north- ern Idaho (Fig. i) is one of the

t wX H i N G T 0 N unique land areas of the world,

Effensb d <R+ and one of its most unusual 4 Yakimat < 5 / features is the snowdrift that

< ver Na!laa e persists on leeward slopes during most of the winter. These drifts

FIG. i-Location of the Palouse. and their effects on the land and

on the agriculture are the theme of this paper. The Palouse is an almost completely dissected plateau, which rises in

elevation from I200 feet at its southwestern tip to about 2800 feet along its eastern border. It has virtually no undissected upland but also is almost without alluvial bottomland. The surface is practically all sloping. The south and west slopes of the hills are universally of more gentle gradient, averag- ing less than 20 per cent and almost never exceeding 35 per cent, whereas the north and east slopes always show a maximum gradient of not less than 40 to 55 per cent. Since there are from several to many hills to a square mile, the general effect from the air is one of dunelike topography (Fig. 2);

in fact, the hills of the Palouse look more like sand dunes than in anv other cultivated area I have observed. The local relief in any i6o acres is from ioo to 200 feet.

A modified maritime climate prevails. The winters are typically cloudy with considerable rain and snow, the summers generally clear with little or no rain. The precipitation ranges from about I2 inches at the southwestern edge to about 22 inches along the eastern border near the mountains. Strong southwest winds, high humidity, and temperatures in the twenties and thirties characterize much of the winter; occasional warm chinook winds bring spells of milder weather.

The soil consists of silt loam and silty clay loam. It has a usual depth over the underlying basalt of I00 feet or more except in the draws, where the depth varies according to the degree of entrenchment. Practically all the land is in nonirrigated cultivation, with wheat and seed peas the main crops.

) MR. ROCKIE is chief of the Project Plans Division, Soil Conservation Service, Pacific Region. He is the contributor of several articles to the Geographical Review, in- cluding "Man's Effects on the Palouse" (January, 1939).



458 THE GEOGRAPHICAL REVIEW

THE SNOWDRIFTS

The snowdrifts that are the identifying mark of the Palouse in winter are the result of several factors. The almost continuous southwest winds tend to lay bare the windward slopes and the hilltops and to pile a deep drift on the leeward slopes immediately below the crest (Figs. 3-6). The altitude is another factor, the high latitude and the resulting low incidence

FIG. 2-Vertical aerial mosaic of a six-by-nine-mile area of Palouse farmland between Pullman, Wash. (at left), and Moscow, Idaho (at right). The light-colored spots and streaks are ridges or hilltops. The white line running north and east of Pullman marks a typically crooked watershed.

of the winter sun a third. Also, simultaneously the winter snowfall reaches its greatest amount and the southwest winds reach their maximum velocities.

The drifts attain fantastic size (Fig. 7), and, since no two hills have the same contour, no two drifts are identical. The snowdrifts may, and fre- quently do, block every road in the Palouse, so that during bad winters constant clearing is necessary. I have encountered drifts three feet deep 30 minutes after the highway was scraped clean. When heavy drifting and heavy snowfall occur together, travel occasionally becomes impossible.

Although the pattern of precipitation is unusually uniform in the Palouse, snowdrifting materially localizes its effectiveness. The bare cultivated hilltops do not have more than half the fall available for absorption, evap- oration, and transpiration; the other half is blown or washed away. At Pullman, for example, where the average precipitation is 20.8 inches, snowdrifting in effect removes the clay hilltops from their natural rainfall belt to a much drier one.



F --

-~~ ~ ?' :

.1~~~~~~~~~~~~~~~~~~~~~~~~~~~~I wp - 1

4.-~-

FIG. 3-Snowdrsfts form behind every hilltop. The drift's outline is determined by the shape of the hill,

by the exact direction of the causative wind, and by the amount of snow.

A Snowdrift ~ - - Pealn B

- - - ~~~~Prevailing south- FPrvingsuh

-4 wesery wnd westerly winds

- I-

- Soil deposition Soil erosion

FIG. 4-Soil erosion and deposition on a typical Palouse hill without snowdrifts (A) and with snowdrifts

(B). Symbols above and below the profile line represent movements due to water and wind respectively. Vertical

scale exaggerated twice.

459



-Fe -5r .4r s;;i;--*<

-4-

FIG. i-View southward from Steptoe Butte, showing Palouse farmland immediately after a snowstorm. Every ridge and hilltop has been blown bare; the number of hills and ridges can be accurately determined by counting the number of black streaks.

-

-~~~~~~W k -

avffi A - ---

r - ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ W

.7'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~S

FIG. 6--View northward from the same hill at the same time, showing a much larger extent of bare land than is visible in Figure s1 owing to the action of the southwest winds.

460



SNOWDRIFT EROSION IN THE PALOUSE 46I

Where drifts pile up, the situation is entirely different. The land directly beneath a 3o-foot drift would, if it could absorb such an amount, receive as much as ioo inches of water (other parts of the leeward slopes receive less extra water). Since absorption is impossible, one or more things happen. The commonest result is a heavy concentration of rilling (Fig. 8). The entire plow layer sometimes gets so oversaturated that a large amount of the soil simply lets go and slides or flows to the foot of the slope. These "slips" or "flows" are generally only as deep as the thawed surface mantle, but they may be as deep as the plow layer, and occasionally they take the subsoil down to three, four, or five feet. Consequently, the slope cannot be farmed when the rest of the field is ready for tilling. If the slip is deep, the slope cannot be farmed at all until a major repair job has been done.

Before cultivation began, bunch grass I8 to 24 inches high covered the entire Palouse. At that time snow did not commonly drift to any extent. When the snow got so deep that the grass was completely hidden, then- and not before-snow began to drift. This conclusion is based on many years of observation of both bare and stubble-covered Palouse hills. Wher- ever good stubble (or grass) stands over winter, drifting is minimized. If the stubble stands as high as the snow mantle, drifting does not occur.

One progressive farmer, who quit cultivating the steep slopes and clay hilltops in his wheat fields and planted them to suitable protective plant covers, made the following statement: "As nearly as I have been able to figure, I find that the 85 per cent of my whole farm that is now left in the regular cropping system is producing about 95 per cent of my old crop at about 8i per cent of my old cost of production. The other difficult IS per cent (hilltops and steep slopes) was producing only 7 per cent of my crop at about I9 per cent of the cost of production on the whole farm. I have a greater net profit now than I was formerly getting from the whole farm without my doing a thing with the discarded acres. And those do have real plus values which could add further to my net profit. The good land of the farm was supporting these low-producing and too expensively operated steep and hilltop acres." The farmer made this statement before the practice of green-manuring with grass-legumes became common. This practice in- creases production generally and might materially alter the percentages.

DEDUCTIONS AND RECOMMENDATIONS

As a result of my study of this problem, I have reached certain tentative conclusions and deductions. They are:

I. Clean cultivation of the Palouse hills is leading to heavy snowdrifting,



, , .. . . ..., ,.#.. . ...

. . . ,. ' X " ; ''N.',I~

NA

FIG. 7-A drift like this, with a depth of nearly 30 feet, creates a real problem for the farmer who crops the field in which it is located. Above the drift the land is dry enough to cultivate in March or April, whereas the drift location may not even be ready to till in early June.

'Ij

FIG. 8-The localization of erosion downslope from the snowdrift can be more effectively photographed after the drift is practically gone. The source, size, and frequency of the damaging rills then become clear. The lower part of such a slope may lose several inches of soil mn a single season.

462



SNOWDRIFT EROSION IN THE PALOUSE 463

which is making the steep leeward slopes increasingly steeper (and therefore decreasingly suitable for farming) because the top of the slope is being built up and the lower part is eroding rapidly away.

2. Cultivation of the hilltops has made the ridgelands drier and therefore less productive of plant growth than they should be.

3. The steep north slopes are so badly eroded downslope from the drifts that heavy farm machinery cannot be safely used on them.

4. It has become increasingly necessary to start farming these slopes many weeks later than the surrounding parts of the fields.

5. Planting the hilltops to some permanent cover materially decreases snowdrifting and at least partly restores to the land the amount of precipitation with which Nature endowed it.

For best land use, the hilltops and steep slopes should be planted to (a) trees, for production of posts, firewood, shelter for livestock, food and shelter for wildlife, and edible nuts and fruits, and for recreational areas, (b) adapted shrubs, for the production of food for man and food and shelter for wildlife, or (c) adapted grasses and/or legumes, for the production of hay, seed, and forage. During the past IS years eleven thousand acres have been thus treated by landowners who have seen the problem, but a hundred thousand untreated acres still remain.



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Snowdrift Erosion in the Palouse