94 THE POPULAR SCIENCE MONTHLY 



unhatched, and where they have been placed in favored locations along 

 the shores of the streams at low stage. We may then only guess at the 

 destruction which must ensue when the entire condition is suddenly 

 and drastically altered by the untimely arrival of the flood. Clear 

 shallow waters, warmed by the sun, are in a brief space of time replaced 

 by deep and turbid torrents, and the very banks and bottoms are torn 

 away or displaced. To fish life another catastrophe has occurred. 



It will not be maintained that any practicable scheme of control, 

 however comprehensive, will prevent altogether the occurrence of high 

 and low stages, but it has been attempted to show that the regulation of 

 the flow of rivers has a very direct relation to the reproduction of fish. 



Without successful reproduction we certainly can not have fish; but 

 the abundance of fish, even under natural conditions, does not depend 

 alone upon successful propagation. The young fish must survive and 

 grow, and for these ends their requirements are similar to those of other 

 animals, namely, food and oxygen, principally. Likewise, just as in 

 the case of other animals, the food is derived ultimately from the es- 

 sential chemical constituents through the intermediation of plants. 

 The rains that wash the soils bring the needed constituents into the 

 streams, but not necessarily in a form available for animal life; for 

 them to become available requires time, sunlight and vegetation. 



It is clear that excessive turbidity and extreme conditions of flood 

 have the most direct bearing upon the conditions of food supply for 

 fish. Not only is this the case, but extreme low stages may have a 

 highly deleterious effect. The first result of the decomposition of or- 

 ganic matters brought into the water is the exhaustion of the oxygen 

 supply, and this may proceed to such a point as to make the environment 

 distinctly unfavorable for any form of aquatic animal life. The begin- 

 ning of mortality among the animals, whether smaller or larger forms, 

 by adding to the amount of decomposing material, only serves to in- 

 crease the rate of deoxygenation of the water and to accelerate the 

 course of destruction. Such a catastrophe can be checked or restricted 

 as to its duration or territory of action only by the development of 

 sufficient plant-life to effect a restoration of equilibrium, or by the 

 diluting and cleansing effect of an increased flow in the stream. Some 

 of the instances not infrequently reported of enormous mortality of 

 fish in portions of rivers just below cities and in times of low water 

 are most certainly due to this very fact of a disturbance of the estab- 

 lished equilibrium between sewage, plants and animals, with a conse- 

 quent mortality that is self-accelerative to the point of inducing a con- 

 spicuous catastrophe. 



Conservation of Favorable Environments 

 There has been developing in recent years almost a new science 

 which deals with the gas-content and the chemical analysis of water, 



