39 



size, that on June 14 of 1,039,619,680, attends the decline of the 

 flood next in importance that culminating May 25 at 13.9 feet; 

 while the third pulse, that on December 15 of 436,535,790, attends 

 the decline of the flood culminating November 25 at 8.7 feet. The 

 hydrograph of 1897 (Pt. I., PI. XI.) is unlike tha^of 1S98 (Pt. I., 

 PI. XII.) in the delay of the so-called "June" rise, which culminates 

 July 5 at 7.5 feet. Its decline runs through the month into August. 

 The diatom pulse attending the "June" rise of 1897 appears about 

 a month later than it did with the earlier pulse of 1898, culminating 

 July 14 at 1 1,459,289,600. A delay in the flood is thus attended by' 

 a delay in the diatom pulse. In 1897 there is no December rise and 

 no diatom pulse of noticeable magnitude, though in 1895, in similar 

 absence of the flood, there is a well-defined diatom pulse. In 1896 

 there is a series of five floods, each involving the early stages of 

 overflow (Pt. I., PI. X.), and on the decline of each occur one or 

 more diatom pulses. 



It is but natural that the greater number of diatom pulses should 

 fall on declining river-levels, since, as I have previously shown, 

 these periods exceed in duration those of rising floods. They also 

 predominate during the prevalence of seemingly favorable temper- 

 atures, and are characterized by relatively more stable conditions 

 in the environment. There is, however, it seems to me, another 

 and more potent reason why diatom pulses appear at such times. 

 It lies in the overflow of seed-beds in the margins of the permanent 

 backwaters and the run-ofl of the plankton which develops there 

 with the fall in levels. This is very apparent to one familiar with 

 the locality. During the decline of the flood the channel current is 

 often diverted in minor lateral channels, such, for example, as that 

 (Pt. I., PI. II.) which courses through Thompson's Lake Slough into 

 Thompson's Lake and out again into the river at its southern end by 

 way of "the swale" and the "cut road." A similar current on the 

 eastern bottoms, which enters partially by way of Mud Lake Slough, 

 rejoins the river through Quiver Lake. These lateral currents are 

 joined by the run-off from overflowed bottoms and adjacent 

 marshes and swamps, all of which, as well as the permanent back- 

 waters thus draining into the channel, breed at such times an abun- 

 dant plankton including diatoms. The contributory function of the 

 backwaters to the plankton of the river proper is thus at its maxi- 

 mum during the decline of the flood. 



