396 
BULLETIN OF BUREAU OF FISHERIES 
From the fieldwork on unpolluted streams in areas where surface erosion was 
not materially influenced by man and where water conditions were otherwise favor- 
able, it has been shown by Ellis (1936a) that the millionth intensity depth (i. e., the 
level at which the light entering the surface of the stream would be reduced to one- 
millionth of its surface intensity) for clear unpolluted streams carrying little or no 
erosion material is 50 meters or more, and that in streams carrying a heavy load of 
erosion silt, like the Missouri River, the millionth intensity level may be reduced to 
less than 100 millimeters. Until erosion is brought under control little can be done 
in demanding a minimum amount of silt and consequently no standard has been 
suggested here, but data from over 6,000 determinations on inland streams show that 
the silt load of these streams should be reduced so that the millionth intensity level 
would not be less than 5 meters, if conditions even approximating those of times past 
when erosion was held in check by forest and grasslands are to be restored, in the 
average inland stream of the United States. The detrimental nature of erosion silt 
as regards fisheries is discussed more fully by Ellis (1936a). 
However, particulate matter introduced by man into streams can be regulated, 
and the detrimental action of various types of suspensoid pollutants is discussed 
under that heading. 
DEPTHS 
As various depths of water are selected by different species of aquatic animals, 
and even by different ages of the same species in many cases, there is naturally no 
single optimum depth of stream water for all types of aquatic life. Depth studies 
of streams and flowing waters, however, bring out differences between waters of 
natural lakes and those of flowing streams and river lakes. 
Current action in flowing streams mixes the water so thoroughly and continu- 
ously that in general the composition of the aquatic environment as presented by 
the water itself varies but little from a few inches below the surface to a few inches 
above the bottom of the stream. The temperature of the water, its turbidity, dis- 
solved solids content, pH, and even dissolved gases content are much the same 
throughout the bulk of the stream at any given station, if there be definite current 
action. Sloughs and other rather isolated lateral areas, particularly those contain- 
ing large masses of aquatic vegetation, are of course excepted, but even in such cut- 
off portions of streams the actual differences in water characteristics as compared 
with the main stream mass are often small. 
Of significance in pollution studies, these depth data show that pollutants may 
be expected to mix rather rapidly and completely with the waters in streams and 
even river lakes unless these river lakes be more than 50 feet deep, and even in those 
river lakes unless the seasonal conditions are such as to promote the stratification 
just described in the deeper portions of such lakes; and that the products of bottom 
pollution will also be distributed throughout the waters of the stream below such 
pollution. 
