x] STREAMS. 25 
northeast of Market Lake, and small tracts of dunes are present else- 
where. In general, however, the soil of the plains is a fine, yellowish- 
white, silt-like material, largely a dust deposit, which mantles the sur- 
face not only on level tracts but covers hills and broad depressions 
alike. This material is similar to the celebrated loess of China, except 
that it usually occurs as a comparatively thin layer, and resembles 
also the deposit bearing the same name in the Mississippi Valley. Like 
each of these formations, it is of exceptional fertility if properly 
irrigated. 
STREAMS.a 
The Snake River Plains lie entirely within the hydrographic basin 
of Snake River, or the Lewis Fork of the Columbia, as it is sometimes 
termed. It is much to be desired that this great stream might be 
named Lewis River, in honor of one of the two bold explorers who 
secured the far northwest for the United States. The Snake or Lewis 
River has its headwaters in Wyoming, near the Yellowstone National 
Park, and drains all of southern and central Idaho. It flows across 
the southern part of the State in a great curve, concave to the north, 
to the Idaho-Oregon boundary, where it turns abruptly northward, as 
is indicated on the map (PI. I). Its mean discharge, as determined by 
the hydrographic division of the United States Geological Survey in 
1897, at Montgomery Ferry, midway between American Falls and 
Shoshone Falls, is 10,064 cubic feet per second; the maximum being 
in May and June, when it reaches about 26,000 cubic feet per second, 
and the minimum in August, September, or October, during which 
months its volume is approximately 4,400 cubic feet per second. b 
In southeastern Idaho many streams join the Snake on the east. Of 
these the South Fork and Blackfoot River are the most important. 
Several perennial tributaries come to the main river from the south, 
such as the Portneuf, Salmon, and Bruneau rivers, besides a number 
of creeks which flow only during the winter or in May and June, when 
the snow on the mountains is melting. All of the tributaries from the 
south are small in volume, particularly in summer. The Bruneau 
would be an important exception to this rule, as it is supplied mainly 
by springs, if its waters were not so largely used for irrigation. 
One of the most significant peculiarities of Snake River is the fact 
that between Henrys Fork, on the extreme northeast, and the mouth of 
Boise River, at the Idaho-Oregon boundary, a distance measured along 
the general course of the river of about 350 miles, it does not receive 
a single perennial tributary from the mountains to the north of the 
a See also p. 159. 
&Such accurate data as are available concerning the hydrography of Snake River and its tributaries 
in the portion of its course bordered by the plains of southern Idaho, may be found in the reports of 
the hydrographic division of the U. S. Geological Survey, particularly in the Twentieth Annual Report 
of that survey, pages 61, 469-491; and in the reports of the State engineer of Idaho, especially the bien- 
nial report for the years 1899-1900, published at Boise. 
