STREAM POLLUTION 
381 
in the river waters do change the pH values of the extremely acid and extremely 
alkaline wastes rather rapidly to the range of the composite pH 6.3 to pH 9.0. 
Reviewing all of the field pH data it may be observed within the general range 
of unpolluted water, pH 6.7 to pH 8.6, that mountain streams, particularly head- 
water streams, are in general more acid than plains or lowland streams. The Columbia 
River data (fig. 14P) bring out this comparison in the bimodal distribution of the 
pH values, since portions of the Snake River flowing through plains and desert 
regions and the mountainous headwaters of both the Snake and Columbia Rivers 
J. MISSOURI HEADWATERS 
24 STATIONS, 25 DETERMINATIONS 
L. TENNESSEE 
116 STATIONS. 316 DETERMINATIONS 
100 
Figure 13 —Continuation of comparisons of pH values, stippled graph A figure 11, being the standard. 
are included in this graph. This same distribution of pH values is also apparent in 
the graphs presenting the findings in the two parts of the Missouri River system 
(figs. 13 1 and 13J). The explanation of these distributions of pH values is clear 
when the carbonates and conductivities of these waters are also considered (v. i. ) , 
as the headwater streams contain much less dissolved solid matter than the plains, 
lowland, and desert streams. The effects of acid pollution are evident in the Ohio 
River (fig. 13 K), and of general pollution and some bog waters in the Atlantic coast 
streams (fig. 11 B). 
The field observations presented here agree in general with the statement made 
by Shelford (1929) that the hydrogen-ion concentration from pH 6.5 to pH 8.5 may 
