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A. VALLEY NEAR THE HEADWATERS OF THE CHARLEY RIVER Showing the effects of local alpine glaciation. U-shaped valley of Moraine Creek at left; morainal

fill in the main valley.

[graphic]

B. UNDIFFERENTIATED (POSSIBLY UPPER SILURIAN) LIMESTONE ALONG THE

WEST BANK OF THE YUKON RIVER JUST ABOVE TAKOMA CREEK

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Circle by way of Chatanika, and this road should be of great benefit to the Circle district, but no steps have yet been taken to connect the Fortymile district with the Tanana Valley.

CLIMATE

The climate of the Yukon-Tanana region as a whole is characterized by long, cold winters and short, relatively warm summers. The extremes of temperature are from 80° below zero in winter to 90° above zero or perhaps higher in summer, with an annual mean temperature of about 24° At Eagle, according to the United States Weather Bureau,1° there are on the average 56 days during the year when the maximum temperature exceeds 70°, 255 days when the minimum temperature is less than 32°, and 120 days when the minimum temperature is less than zero.

The mean maximum temperature from May 15 to September 15 is about 65° and the mean minimum about 40°; the mean maximum from November 1 to April 1 is about 10° and the mean minimum for the same period about – 15°. Commonly, the alluvial deposits are permanently frozen to great depths and thaw only a few feet at the top during the summer. A marked exception to this condition exists along the banks of the larger streams, where circulating ground water has in places thawed the ground for several hundred feet back from the river banks. In winter ice freezes on the lakes and quiet ponds to a depth of 5 feet or more. The permanently frozen ground is believed to be evidence of a previous geological epoch, in part Pleistocene, during which the regional climate was even more frigid than at present. This deep frost may, therefore, be regarded as an inorganic fossil record of a preexisting climatic condition.

The larger streams, such as the Yukon, usually begin to freeze over about the middle of October, and the ice breaks up about the middle of May. The smaller streams freeze earlier in the fall and open later in the spring. In the higher country killing frosts are rare in midsummer but begin in the middle of August and sometimes continue throughout May. In the lower country, as along the Yukon, the season free of frost is somewhat longer.

The average annual precipitation at Eagle, based on observations made over a period of 18 years, is 10.4 inches. The average winter snowfall, based on observations over a period of 12 years, is 51 inches. Without doubt, both rainfall and snowfall are somewhat greater in the mountains away from the Yukon, but the region as a whole is semiarid. On account of the frozen substrata, the circulation of deeper ground water is restricted. A heavy surficial covering of sphagnum moss tends to limit the run-off and to hold much of the precipitation near the surface. This wet ground favors the growth of vegetation, so that the semiarid nature of the region is not at once apparent to the casual observer.

10 Summary of the climatological data for Alaska, by sections: U. S. Weather Bureau Bull. W, 2d ed., vol. 3, 1926.

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VEGETATION

The common trees are spruce, cottonwood, and white birch, of which spruce is the most plentiful. Black birch and tamarack are also found. Spruce trees a foot or more in diameter may be seen in the bottoms of the larger valleys, but on the ridges and spurs the trees are small, both in height and girth, though in places very densely spaced. Timber line is at about 2,500 feet on most of the ridges, but on limestone ridges and in the heads of the larger valleys it may be several hundred feet higher.

In addition to the trees, a variety of other vegetation thrives. Along the smaller streams grow dwarf willows and alders, which in some places are almost impenetrable, and on many of the spurs, just below and above timber line, the dwarf black birch forms a dense undergrowth. Many smaller plants and shrubs also abound. Partial lists of these plants have been made by different observers, usually in connection with some other work, such as geology, but no comprehensive study of the flora has yet been attempted. Several species of moss are very common, covering the ground in the lower valleys and extending well up on the ridges. Toward and above timber line the lichens become more prevalent. These constitute one of the sources of food for the herds of caribou that roam through the country, particularly in winter, when the brush and other flowering plants are dormant.

Grass for horses is abundant in most of the river valleys and on the lower spurs. On the upper slopes of many spurs a moderate quantity of bunch grass suitable for stock may be found. Along the river bars a horsetail rush grows in great abundance, and locally the pea vine; both of these, particularly the pea vine, are eaten by horses. Stock can therefore find pasturage during June, July, and August.

Many varieties of berries grow wild, of which the blueberry and low-bush cranberry are the most abundant and useful. Red raspberries, red and black currants, high-bush cranberries, and other berries are also found. At Eagle, Circle, and other places along the river gardens are planted, and all the hardy vegetables, including potatoes, turnips, cabbage, lettuce, rhubarb, beets, carrots, and radishes, are grown without difficulty.

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