384 
BULLETIN OF BUREAU OF FISHERIES 
osmotic pressure on the aquatic organisms living in the water, and many of these 
compounds are physiologically active, so that fresh- water fishes and other animals 
living in these streams have become adapted to the physical and physiological actions 
of this salt complex. 
Small variations in several of these salts may cause small variations in the 
species composition, particularly the invertebrates, of the faunae at any given station. 
Most aquatic species, however, will tolerate changes of considerable magnitude in 
the relative amounts of these salts normally present in flowing waters, if the very 
small total amount which is usually present be not exceeded. For example, the fixed 
carbonates in the upper Tennessee River were found to vary from 0.4 to 30 cubic 
centimeters per liter (computed as C0 2 by volume) without affecting the general 
composition of the aquatic fauna, and from 0.3 to 55.5 cubic centimeters in Spider 
Creek, a tributary to the Wabash River. 
The specific quantities of most of the substances comprising this salt complex 
are not so important as the total quantity of soluble matter involved (see section on 
osmotic pressure), since even the small quantities of these compounds present in 
ordinary soil run-off are in excess of the physiological needs of most fresh-water 
organisms. As in general these substances are ionizable, measurements of the spe- 
cific. conductance of a large number of polluted and unpolluted waters were made. 
The summarized data are presented in figure 15. 
These determinations of specific conductance of inland fresh waters show that, 
excepting the streams in the plains and desert regions, the specific conductance of 
those portions of inland streams and rivers which were supporting good, mixed fish 
faunae in general lay between 150 and 500 mho X 10 -6 at 25° C. 
This uniformity of water composition in flowing streams holds even in the very 
deep holes of rivers. Two cases — Pan Eddy, in the Tennessee River, and the deep 
hole off Grand Tower, 111 ., in the Mississippi River — are presented as typical in 
table 1. 
Only in the very deep portions of river lakes, such as Lake Wilson in the Tennessee 
and Elephant Butte Reservoir in the Rio Grande, is there any marked stratification 
of the waters. In the deeper portions of some of these impounded waters (see table 4) 
definite thermal stratification, with the attendant changes in turbidity, dissolved 
oxygen, pH, and other physiochemical features of the water, develop in midsummer 
at levels variously determined by the general climatological features of the region 
and the level and amount of draw-off as made for the needs of industry and navigation. 
In such river lakes a warm, more turbid stream — the hyperlimnorrheum — flows over 
a colder, clearer lake — the hypolimnion — with a rather well-delimited thermocline 
lying between these. This condition of stratification obtains, however, only during a 
portion of the warm season. Specific data for one station on Lake Wilson (see table 4) 
show such stratification of the waters when it was at its height. During the colder 
portions of the year there is a rather complete mixing of the waters at all depths, even 
in Lake Wilson. Details of these hydrobiological data of various river lakes are 
presented elsewhere. 
The specific conductance of mountain stream waters was generally in the lower 
part of this range, unless excess carbon dioxide were present. However, in the streams 
of the western plains and desert areas, particularly those carrying the more alkaline 
