146 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
formerly luxuriant meadows of wild grasses and their replacement by 
sage brush. The rapid erosion in progress is destroying much of the 
finest land in the mountain valleys lying north of the Snake River 
Plains, and its prevention is an exceedingly important problem, which 
demands immediate attention. 
The beginning of this recent erosion is correlated with the introduc- 
tion of sheep in large numbers into the country, and is but another 
phase of the widespread destruction for which sheep raising on a 
large scale is responsible. The cause of the washing is mainly the 
destruction of the once abundant bunch grass by the close grazing of 
sheep. 
It might, perhaps, be thought that if sheep raising could be discon- 
tinued or properly controlled the destructive washing of the land inci- 
dent to it under certain combinations of soil and climatic conditions 
would be arrested, but the process of erosion once started is difficult 
to stop, and will continue even if the sheep were removed. Success- 
ful attempts to check the work of rills where they are rapidly deepen- 
ing their channels have been made by a method that is simple and 
efficient. Near the barrack at Boise small dams placed in the streams, 
with suitable precautions against erosion on account of the plunge of 
the water, have led to the deposition of gravel and sand and caused it 
to form broad fields, which will no doubt in time be cultivated. 
Under this method, however, erosion still progresses above where the 
water is ponded and the destruction continues, and will necessitate the 
building of additional dams lower down the stream courses or an 
increase in the height of those already constructed. Probably the 
best method of checking the work of the rills and brooks is to plant 
trees in them, and especially the planting of eottonwood, willows, etc., 
in belts across the channels where widest. By systematic attempts of 
this character much of the damage that threatens can be cheaply and 
efficiently checked. 
One result of the cutting ot channels through previously completed 
flood plains which are under cultivation, as in the example shown on 
PI. XXV, is the more complete subdrainage of the portions of the 
fields remaining, thus necessitating additional irrigation. In many 
such instances the construction of willow dams sufficient to check the 
flow of the water and cause deposition would also lead to lateral per- 
colation from the streams, which would be equivalent to subirrigation. 
The destruction by recent rill erosion is not confined to the upland 
valleys, but, by reason of the immense amounts of gravel and sand 
swept down to the larger streams, causes them to spread out sheets of 
this material on their flood plains, which bury the vegetable mold and 
greatly impair and even destroy the agricultural value of the land. 
