NORTHWESTERN LAKES OF THE UNITED STATES. 
69 
very near freezing by the long cold winters. On the other hand, Lake Whatcom 
is near sea level and so close to the Pacific coast that it probably does not reach 
the temperature of maximum density. The higher temperature of the bottom 
water, 5.4° C., seems to substantiate this. The bottom waters of both lakes are 
well supplied with oxygen, indicating a fall overturn and circulation. 
It should be noted that Bear Lake, with a depth of 56 m., has more oxygen at 
the bottom than at the surface, while no other lake less than 95 m. deep had this 
large supply of oxygen at the bottom. This may be accounted for by the high 
altitude (2,216 m.), which shortens the summer season. 
The percentage of saturation of oxygen in the very deep lakes varies a little 
from the others. Since the surface water is usually colder there is a smaller growth 
of phytoplankton, and the one set of determinations on Pend Oreille is the only 
one that shows a marked supersaturation in the epilimnion. In most cases the 
water at the thermocline is very nearly 100 per cent saturated. At the bottom, 
although there is more oxygen present, the percentage of saturation is not high. 
Lake Chelan has 90 per cent saturation (calculating the saturation from the alti- 
tude of the surface, page 114), and all the others haveless than 90 per cent saturation. 
Birge and Juday (1914) have studied the Finger Lakes of New York and have 
found similar oxygen conditions in Seneca, Cayuga, Skaneateles, and Canandaigua, 
which are 173, 122, 83, and 80 m. deep, respectively. 
In the deeper lakes the bottom temperature usually approaches 4° C., the 
temperature of water at maximum density at atmospheric pressure. (See Crater 
Lake, p. 107.) The depth of a lake changes the position of the thermocline very 
little; that is, the depth of the epilimnion is no greater in a deep lake if the size and 
the protection from wind are the same. It is the hypolimnion that is increased in 
depth and volume by the increased depth of the lake. Since it takes a longer time 
to warm the larger volume of water before the thermocline is formed in the spring 
and because of a small diffusion of heat through the thermocline, the temperature of 
the epilimnion of a deep lake is usually a little lower than that of a shallow one. 
This lower temperature of the epilimnion retards the growth of algae; therefore there 
is not as much algal material to decay in the lower water. The larger volume of the 
hypolimnion furnishes a proportionately larger supply of oyxgen to carry out this 
decomposition. 
The plankton algae are found chiefly in the warm water of the epilimnion. When 
they die, they settle slowly in the warm water; but when they reach the cold water at 
the thermocline, which has a greater density, their settling is retarded, if not stopped 
entirely for a time. This has been shown by investigations in some of the Wisconsin 
lakes. A definite decrease in the oxygen has been noted just below the thermocline, 
which indicates more decomposition at this depth than farther down. The cold 
water of the hypolimnion also retards both the speed of settling and of decay. 
The viscosity of the water must also play a definite part in this speed of settling, 
because the viscosity of the cold water of the hypolimnion is almost twice that of 
the epilimnion. The viscosity or lack of fluidity retards the settling of these algae, 
because their specific gravity is only a little greater than that of the water. In 
very deep lakes the combination of the increasing density and the viscosity may 
retard the settling of the decaying algae, so that they are largely decomposed before 
