8 BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 
ditions of flow are suited for this purpose there is no need for irri- 
gation. 
Under the conditions existing in this valley the most important 
function of the watercourses is the removal of excess water. This 
constitutes a combined problem of drainage and flood control which 
is of vital interest over practically the entire watershed. This 
problem is discussed later in the report. 
The Red River and its tributaries are the source of the water 
supply for nine cities and towns having a total population of about 
69,000. The most important considerations in the use of streams 
for municipal water supply are the quantity and quality of the water. 
It is not sufficient that the total amount of water passing through 
the stream channel during the year be great enough to supply de- 
mands; the flow must be so distributed, either naturally or by arti- 
ficial means, that at no time will it fall below a minimum consistent 
with safety. To insure a sufficient degree of dilution of impurities 
and provide for future increased demands, this minimum flow should 
always be many times greater than the rate at which the water is 
withdrawn from the stream for use. In populated districts stream 
water is seldom of such quality that it can safely be used for domestic 
purposes without treatment.. Where the streams furnish the water 
supply the flow is abundant except at certain points on the Red Lake 
River where the flow during exceedingly dry seasons_becomes so 
small as to occasion concern over the concentration of impurities. 
The increase in minimum flow planned for this river as a part of the 
Red Lake project would improve this condition. In some of the 
smaller tributaries the dry-season flow is so slight that water supply 
from this source is impracticable. In most cases the water taken 
from the streams is passed through filters which remove the greater 
part of the suspended impurities, and in the upper parts of the 
streams such* treatment is sufficient to render the water fit for use. 
However, the water of this drainage basin is very " hard," owing 
to salts leached from the ground, and can be much improved for 
domestic use by softening through chemical treatment. In the lower 
parts of the streams where the accumulation of impurities from 
above is greater, chemical treatment also is necessary especially dur- 
ing low-water periods, to assure absence of organisms dangerous to 
health. Fargo and Grand Forks are now operating plants for such 
treatment of the water, and it is probable that, as the cities along 
the streams grow and become more numerous, chemical treatment 
will be commonly resorted to. 
The streams in the Red River basin are more used as a means of 
sewage disposal than for Avater supply. By dilution and by 
the action of natural purifying agencies, obnoxious and dan- 
gerous effects of this contaminating content are eliminated or 
