398 BULLETIN OF BUREAU OF FISHERIES 
Table 5. — Water temperatures of inland streams, exclusive of headwaters in mountainous regions, 
during warm season, June to September, inclusive, 1930-35 
River system 
Num- 
ber of 
stations 
Num- 
ber of 
cases 
14.0°- 
17.9° C. 
18.0°- 
21.9° C. 
Percent 
22.0°- 
25.9° C. 
of cases 
26.0°- 
29.9° C. 
30.0°- 
33.9° C. 
Above 
34° C. 
Mini- 
mum 
degrees, 
Centi- 
grade 
Maxi- 
mum 
degrees, 
Centi- 
grade 
Composite 1 .. 
726 
4,545 
13 
16 
35 
31 
5 
m 
15.0 
36.6 
Entire Mississippi system ... _ . 
447 
2, 887 
12 
12 
46 
27 
3 
0 ) 
15.0 
35.2 
Coastal streams into Gulf of Mexico 3 ._ 
39 
1,367 
7 
14 
32 
36 
9 
2 
16.8 
36.6 
1 Includes flowing streams at stations at which fish were found in Mississippi, Missouri, Tennessee, Ohio, coastal streams 
flowing into Atlantic Ocean and Gulf of Mexico, Rio Grande, Colorado, and Columbia system. 
1 Less than 1 percent. 
3 Exclusive of Mississippi system. 
Considering the composite in table 5, it may be seen that in spite of the high air 
temperatures which prevail in many parts of the United States during the summer 
months, often for periods of days, stream temperatures above 34° C. are not common, 
and that 95 percent of the 4,545 cases listed in table 5 lie below 30° C. On the other 
hand, 66 percent of these 4,545 cases lie between 22° and 30° C. For comparison 
with the composite in this table, data from the entire Mississippi system and from the 
coastal streams of the Gulf States, Texas to Florida, inclusive, are also presented. 
In the Mississippi system 73 percent of the cases fell between 22° and 30° C-., and 68 
percent of the cases in the Gulf States have the same range. 
In evaluating biochemical oxygen demand, dissolved oxygen content, and 
specific toxicity values for pollution studies therefor, a water temperature range 
during the warm season from 15° to 36° C. may be expected in the average inland 
stream of the United States, with the probability of 66 percent or more that these 
water temperatures will lie between 22° and 30° C. During the other portions of 
the year — that is, outside of the warm season — the water temperatures of these 
streams in localities north of the freezing line will fall, with 0° C. as the limit. Our 
field data also show that south of the usual freezing line — that is, in streams of 
southern Texas, Alabama, and Florida — temperatures as low as 10° C. are not 
unusual during the midwinter season. 
The true headwater streams, particularly those in mountainous regions, present 
a different temperature range even during the summer months. The maximal tem- 
peratures for various streams which could be definitely classified as mountainous 
headwaters, as found in our field studies, were rarely above 10° to 15° C., with the 
average values lying around 10° C. or even lower. 
BOTTOM CONDITIONS AS AFFECTED BY STREAM POLLUTION 
In most stream-pollution studies, conditions at the bottom must be considered, 
since the bottom of the stream contributes a considerable part of the food of stream 
fishes through the organisms of the bottom fauna which are either eaten directly by 
fishes or which are included in the food chains of various species of fishes. Besides, 
portions of the bottom usually in the shallower parts of the stream provide nesting 
sites for certain species. However, as the stream bed may change from rock and 
coarse gravel to fine clay in the course of a few hundred yards, and as there is a wide 
variation in habitat preference of the many desirable organisms which inhabit stream 
