BIOLOGICAL EXAMINATION OF WATER BODIES 



Weed of Considering Entire Picture 



It seems worth while to review the general method of collecting 

 data on stream conditions that should be pursued in actual field 

 work. One cannot emphasize too strongly that a fair judgment 

 concerning conditions can be obtained only by taking into considera- 

 tion the entire picture which presents itself to the observer. Single 

 elements of the aquatic situation may be modified by one circum- 

 stance or another, and the student who relies upon any individual 

 factor to determine the value and character of the water area will 

 be frequently led astray. If one starts with the point at which the 

 stream of waste empties into the natural water channel and follows 

 down stream he will observe that, especially if the effluent is colored, 

 it is easy to trace the gradual mixture of the waste with the stream 

 water as it spreads itself towards the opposite bank as well as down 

 stream. The bottom is often colored and the waste furnishes float- 

 ing particles which, in addition to the color tone already mentioned, 

 aid the observer in tracing the spread of its influence over the water 

 body. These general appearances are readily followed at ordinary 

 stages of water. They become very conspicuous during low water 

 conditions, and are more or less concealed in times of high water 

 or when the stream is turbid with the surface run off that is carried 

 in after storms, One can follow out in the way just indicated the 

 area brought under the influence of the effluent, and to determine 

 the biological effect one turns next to the study of the living organ- 

 isms within the area involved. It has been customary in the past to 

 base conclusions upon observation of the fish themselves, but one 

 should not and cannot place dependence upon this or any other 

 single test object. As a means for determining the suitability of 

 the water body for the existence of fish life and its favorableness 

 for the multiplication of fish species, it is better to study the small 

 organisms rather than the fish themselves. The fish are evidently 

 less subject than are the smaller organisms to the control of the 

 immediate environment and better able to change continuously 



