8 
July contains about twice as many as any of the lists of those 
found at the same time in the lakes of northern Germany or in 
our own Great Lakes. 
The bluff beyond the bottoms to the west is higher than 
that on the east, and usually of a very different character. 
Strata of carboniferous rock, sometimes containing veins of 
coal, outcrop locally near its base, while the higher slopes are 
formed of yellowish clays, ditched and gullied by the rain, with 
occasional small streams flowing through gorge-like valleys 
from the level uplands of the country farther west. 
The description thus far given applies to the lower stages 
of water only. When the river is at flood the entire bottom¬ 
land from bluff to bluff is often wholly under water, lakes, 
streams, and marshes being then confounded in one unbroken 
sheet from three to five or six miles across (Plate XIX.). As 
the river level varies some eighteen feet between high and low 
water mark, it may reach in its deepest part a depth of nearly 
thirty feet. These periods of inundation are very commonly 
two in a year, one beginning in late winter or spring with the 
melting of the snows, and the other coming most frequently in 
June or July, as a consequence of the early summer rains. The 
rise at either or both these periods is occasionally so small that 
no very marked effect on the biology of the river is produced. 
It was, in fact, fortunate for our operations that the first two 
years of our occupancy of the Station were marked by this com¬ 
parative uniformity in the river level. Observations and collec¬ 
tions made at this time have given us a fairly steady biological 
base line, by comparison with which variations in other years 
may be detected, due to extensive overflow and subsequent re¬ 
cession of the waters. 
The plan and purpose of our work was such as to make it 
necessary that we should choose a number of regular stations— 
called substations in our reports—at which collections should 
be made and observations placed on record at regular periods 
for the entire year, and one year after another. These sub¬ 
stations, thirteen in number, were chosen to represent the great¬ 
est variety of biological situations which the territory within our 
reach would permit (see map, Plate I.). They have been suffi¬ 
ciently characterized in the introductory part to a report by the 
Station Entomologist, Mr. C. A. Hart, on the entomology of the 
