134 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. I 
broader tract of tiat land to cross before reaching the main waterway. 
The streams from the southeast, coming from high mountains, are 
well supplied with material in suspension, and have been enabled to 
upgrade their courses across the comparatively narrow tract of flat 
country, and thus reach the main channel of discharge without being 
ponded. Perhaps the best illustration of this, and the only one which 
can here be noticed, is furnished by the South Fork of Snake River. 
As may be seen on inspecting the map forming PL I— or better, the 
General Land Office map of Idaho, from which it was in the main 
compiled — the South Fork on emerging from the mountains, to the 
east of Market Lake, splits up into a number of distributaries, which 
join the North Fork independently. The meaning of this is that the 
South Fork, being overloaded, has built up an immense alluvial fan on 
the border of the Snake River Plains, on which it subdivides. The 
conditions, as previously remarked, are unusually favorable for irri- 
gation, and the natural subdivision of the river has been increased in 
recent years by the construction of canals. The alluvial fan built by 
the South Fork has not only deflected the North Fork and caused it to 
make a curve to the west, but has checked its flow and compelled it to 
drop the material it was previously enabled to carry and so assist in 
forming a broad alluvial deposit. The North Fork has been partially 
dammed by the material deposited in its course, and now widely over- 
spreads its banks during high-water stages. This tendency to form a 
lake was greater before the construction of extensive irrigating works 
and railroad embankments than it is now. for a lake was formed each 
year, which during certain periods was perennial. This Market Lake, 
as it was named early in the exploration of Idaho on account of the 
abundance of game in its vicinity, in now prevented from becoming 
flooded, and its bed is under cultivation. 
Much more space could be given to the study of the upgrading 
being carried on by the streams that join the Snake from the southeast, 
but enough has probably been stated to illustrate the marked contrast 
that exists between them and the "lost river" on the western portion 
of the basin which they enter. The conspicuous difference in these 
two representative instances, it will be seen, is one of degree. Of the 
many factors involved in a river's development, a few or perhaps some 
definite one controls the result in certain instances, while others become 
prominent and perhaps assume the lead in other cases. 
In reference to the process of upgrading, it will be rightly assumed 
from the statements already made that all the streams emerging from 
the mountains onto the lava-formed portions of the Snake River Plains 
are now or at some time in their history have been engaged in spread- 
ing out gravel, sand, silt, etc., over the surfaces of the nearly flat lava 
sheets that they have to cross. In this manner much of the soil of 
the plains has been formed. 
