124 
BULLETIN OP THE BUREAU OF FISHERIES 
It is well to recall that most fresh-water organisms can utilize the water only 
after it has fallen as rain and before it is washed into the sea or evaporated. To 
illustrate the point about to be made, we may take an extreme and hypothetical case. 
It is obvious that a coastal creek from which the water ran into the ocean wit hin a 
coup’e of days of its fall could not have much fresh-water floating life because there 
would be no time for it to develop. Any retardation of this process of carrying 
rain to the ocean increases the chance for these animals to reproduce their kind. 
Accordingly, any checking of river currents should be desirable from this standpoint, 
although, if carried too far, new and unfavorable factors might appear. It is probable 
that this checking of progress to the ocean is the chief reason why lakes are richer 
than rivers. The river is fed constantly, both directly and indirectly, by newly fallen 
water. The waters of lakes have had a longer history from the time when they were 
in the form of falling raindrops. 
Another reason for expecting lakes to possess more organic life is that they 
usually have substantial amounts of rooted vegetation, whereas rivers have but little 
of this (Kofoid, 1903, pp. 236-252) ; and it is rooted plants rather than drifting plants 
that enrich the water from the soil (Pond, 1905, pp. 522-525). Evaporation may be 
of some importance because it replaces, to the extent to which it occurs, run-off to 
the sea, and thus reduces by that amount the sweeping into salt water of river life; 
evaporation is greater on lakes (Rafter, 1903, pp. 23 and 39ff) probably because 
the water is exposed longer and because of the freer action of wind on the water. 
One other point is worth noting. The water of a river does not move forward 
in a simple column but is retarded by the sides and bottom and usually by the air. 
This retardation of the whole perimeter of a cross section of a river causes extreme 
irregularity in the character of the movement. The result is complicated by many 
factors, but it may be said that in a general way a river rolls forward like a wheel, 
or rather like two sets of wheels (Gilbert, 1914, pp. 248-249), and any given particle 
of water must be now and again swept from the surface to the bed and up again. 
It is necessary to diverge a little and recall that green plants are the ultimate food 
of all creatures, and that they can grow only in sunlight. In the water these plants 
are represented by (among other things) minute algae, which constitute an important 
and perhaps the principal ultimate food of aquatic life. As they can grow only in 
sunlight, it is probably exceedingly difficult for them to carry on life processes while 
they are whirled frequently from sunlit areas to the deeper and almost sunless regions 
of the Mississippi River. Whether or not the interruptions as such are harmful, the 
reduction of sunlight certainly is. 
As the result both of observation and of inference, it has been the conclusion of 
some investigators that the plankton of a river is produced largely in the tributary 
waters and is carried into the river, a conclusion that links well with the observation 
that currents are unfavorable to the production of plankton. 10 Allen, for example, 
after extensive studies of the plankton of the San Joaquin River in California, says 
that “water currents above a very moderate speed are distinctly inimical to plank- 
ton development.” 11 (Allen, 1920, p. 124.) Galtsoff (1924, pp. 412 and 413) gives 
some observations to show that there is pronounced plankton development in places 
in the Mississippi River, independent of the contribution of tributaries. It is evi- 
10 For a discussion of this question, see Galtsoff, 1924, pp. 411-415. 
“A similar conclusion: was stated by Schroder in 1899, after studies of phytoplankton on the Oder River, Germany, as men- 
tioned by Galtsoff (1924, p. 413) and adopted by Steuer as Schroder’s law. 
