398 
BULLETIN OF THE BUREAU OF FISHERIES. 
Where currents are found, the water, generally speaking, presents conditions 
adverse to the development of microscopical fauna and flora. The plankton or- 
ganisms are therefore usually more abundant in slow-flowing streams and in stag- 
nant places than in swift waters. The velocity of current is the principal factor 
that affects the organic life in the river. If the current becomes too swift, as at 
rapids or water falls, the amount of plankton decreases considerably. Such a 
condition is found in the Mississippi River above Rock Island Rapids, where the 
amount of plankton carried is four times as great as below the rapids. Sixteen miles 
of rapids destroy three-fourths of* the microscopical population. 
The stage of the river has the same effect on the river plankton, although the 
reason is different. At a high stage the river water mingles with barren storm 
waters and the plankton is washed away. Therefore, every sudden rise of the river 
carries away its plankton population. The instability of the hydrographic condi- 
tions, so characteristic of river regimen, strongly affects the productiveness of the 
river in plankton. The greater the instability of the river conditions the less the 
amount of plankton. Conversely, when a period of low water lasts for a long 
time the plankton may become very abundant. 
The results obtained in the investigation made during a comparatively short 
time in summer are not sufficient to show what relations exist between the dif- 
ferent parts of the river in other seasons. The present data refer only to the warm 
season and the low stage of the river. During the course of the investigation some 
points were visited twice; this makes the conclusions concerning the plankton re- 
sources in those parts of the river more reliable. 
It does not seem necessary to publish here all the records of the examination 
of each plankton sample. The results of this study are presented in Tables 23 and 
24, composed of these original records. The plankton data presented in these 
tables have been summarized and are given in different columns corresponding to 
the following subdivisions of the river: ( a ) The river from Burlington, Iowa, to 
Rock Island Rapids ; ( b ) above Rock Island Rapids, from Le Claire, Iowa, at the 
head of the rapids, to Reads Landing, Minn., just below Lake Pepin; (c) above 
Lake Pepin, from the head of the lake to Hastings, Minn. The composition of the 
plankton of Lake Keokuk is given in two columns, because there was a difference 
between the upper and lower parts of the lake. All the data refer to the main stream 
of the river. The composition of the plankton collected in the sloughs, bayous, and 
among the water plants, and the data obtained at the end of September during the 
rise of the river, are presented separately. 
The symbols in the columns indicate the relative frequency of the organisms, 
as follows: ©, very abundant; 3, abundant; 0, frequent; O, scarce; O, very 
scarce; absent. 
