As the flood recedes, relict pools on the bottom-lands and along 

 the margins of the permanent backwaters are formed, in which the 

 conditions favoring sporulation or other means of providing for 

 resuscitation are to be found. The emerging bottom-lands thus be- 

 come the seed-bed for starting a new cycle of diatoms whenever flood 

 conditions return. In the river, on the other hand, the conditions 

 for sporulation are not so favorable, and the current tends to carry 

 away such resting stages as may be formed. The observed facts 

 regarding the distribution of diatoms and the examination of the 

 conditions under which these pulses occur thus alike yield corrob- 

 oration of the view that floods are potent factors in determining the 

 occurrence of diatoms in fluviatile waters, especially where back waters 

 are extensive. 



The nature of the action of floods is in some respects similar to 

 that of the overturning of the water which occurs in lakes when the 

 point of maximum density, 39.2, is passed in either direction. In lakes 

 of some depth the vertical circulation of so large a volume of water 

 results in a stirring up of the bottom deposits containing the resting 

 stages of diatoms, so that they are brought again into increased 

 light and to better aeration. Whipple ('94) has emphasized the 

 importance of this overturning in starting the growth of diatoms. 

 In our shallow waters this physical phenomenon is of less impor- 

 tance than in the deeper waters of the lake or reservoir. The vol- 

 ume in circulation is smaller, though some compensation for this 

 may exist in the possibility of repeated over turnings with fluctua- 

 tions in temperatures at the critical stage. The existence of cur- 

 rents, the movements of fish, and the roiling effect of strong and 

 long-continued winds upon our shallow backwaters, combined with 

 the fact that much of the seed-bed area of overflow is dry land at 

 the time of the autumnal overturning, all serve to minimize the 

 effect of this overturning in our waters upon the growth of diatoms 

 in the plankton. The spring overturning occurs early in March, 

 and in 1896, 1898, and 1899 a slight pulse not exceeding an increase of 

 100 per cent, follows the overturning within an interval of a fortnight. 

 The vernal pulse is about two months later than the overturning, 

 and the relation of this to the overturning does not seem to be inti- 

 mate. The autumnal overturning occurs towards the middle or end 

 of November, and in 1895, 1896, and 1898 the hiemal pulse of 

 December follows close upon it, within two, or at most three, weeks. 



