IRRIGATION REQUIREMENTS OF THE GREAT BASIN 5 
exceed the inflow, and an intermittent lowering of the surface re- 
sulted. All that now remains of this great lake, which was 250 miles 
long, 180 miles wide, and covered an area of 5i/o million acres, are 
a few relatively small, shallow lakes and several sinks. A large part 
of the area irrigated at present in both Utah and Nevada is located 
in the beds of these ancient lakes. 
GENERAL CHARACTER OF THE SOILS OF THE GREAT BASIN 
The soils of the Great Basin are probably more variable than those 
of any other large land division of the western part of the United 
States. The chief danger resulting from the irrigation of these 
soils is their tendency to become .water-logged or alkaline. 
Residual soils. — Weathering and decomposition of the rocks in 
place have led to the formation of a widespread variety of the soil 
material, confined mainly to the hills and mountains. These soils 
are often shallow and contain an excessive quantity of rock frag- 
ments or areas of rock outcrop, and, owing to rough topography 
and their elevated position with respect to sources of water supply, 
are generalty nonirrigable. These conditions render most of the 
residual soils of the Great Basin suitable only for grazing. 
The soils of the valleys and plains are of various origins and 
frequently have been transported long distances by wind or water or 
both. In many areas the soils have been laid under water. In some 
instances the lakes formed in past geological periods had two ex- 
istences, with a dry-lake period between, resulting in a further com- 
plexity of the subsoils. The lake-formed benches have been cut 
through by streams and the valleys modified by fan-shaped deposits. 
In the following paragraphs the valley soils have been grouped 
according to their formation. The adaptability of each type of 
soil to irrigation is briefly outlined. 
Lake-laid soils. — These soils were formed under water and are for 
the most part rather dense, with poor drainage, and liable to con- 
tain an excess of alkali. They are usually deep, and while they may 
be rich in plant food are hard to handle under irrigation. 
Alluvial fans. — These are the deposits of torrential streams and" 
frequently cover lake-laid material. They are of irregular shape 
but of favorable topography and fairly smooth. The material 
varies from gravel and bowlders near the base of the mountains to 
loam or clay at the outer edge of the fan. The soils are of various 
depth, usually productive, well drained, fairly free from alkali, and 
are readily irrigated. 
Valley fills. — These soils are also largely the result of stream 
action. They are mostly smooth, deep, and productive, but are 
deficient in drainage and liable to become alkaline. As a rule, they 
are near a water supply and easy to irrigate. They vary in texture 
for the most part from sands to clays. The valley fills, together 
with alluvial fans, form by far the greater part of the irrigable area 
of the Great Basin. 
River flood plains. — These result from late river deposits or river 
erosion and deposition. Such soils are mostly well adapted to irri- 
gation but liable to become water-logged and alkaline. The surface 
is usually level and smooth and requires little preparation for 
irrigation. 
