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., Pl. XI.) is unlike that of 1898 (Pt. L., 
Pl. 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 11,459,289,600. 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., Pl. 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-cff 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., Pl. 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, 
.Tejoins 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. 
