SWIFT STREAMS 93 
3. SPRING BROOK COMMUNITIES 
(Stations 10 and 11; Table XIX) 
In glaciated areas many of the streams are fed by springs which 
have not been produced by erosion, but are the result of porous and 
impervious layers of till arranged as in regions possessing artesian wells. 
The presence or absence and numbers of animals in a spring depend 
largely upon the chemical content of its water. Spring waters commonly 
have insufficient oxygen to support animals and at the same time may 
contain sufficient nitrogen and carbon dioxide to be detrimental if not 
fatal to animals. The mineral matter in solution may be large in 
quantity and in some cases poisonous also. As the water flows away 
from the spring it becomes aerated and diluted with surface water so 
that the animals of the spring brook can live init. Spring consocies differ 
in different springs because of variations in the character of the water. 
In an area where there are springs, they are usually numerous. 
The little brooks unite to form larger streams. Typically, such streams 
may not be larger than intermittent streams, but a nearly constant 
flow at all times of the year is one of the characteristic conditions. 
Pools and riffles are not so well defined, but contain some small fishes. 
The watercress grows abundantly at the sides of the stream and affords 
a lodging-place for aquatic animals not furnished so abundantly by 
young streams of other types. The water is colder in summer and 
warmer in winter than in other streams. 
Spring brook associations.—Among the watercress are the amphipods 
(Gammarus fasciatus), the larvae of Simulium attached to the leaves, 
beetles, dragon-fly nymphs, and young crayfishes. Here are also found 
occasional snails (Physa gyrina). The species of the cress association 
are nearly all found under stones or on stones in the riffles. On the 
stones are Simulium larvae and Hydropsyche (95), the net-building 
caddis-worm (Figs. 39, 40, p. 96). Under the stones are the nymphs of 
the May-fly (Baetis and Heptagenia), the larvae of flies and midges 
(Chironomus, Dixa, and Tanypus), the brook beetles (£lmis fastiditus) 
(Fig. 47, p. 98), and occasional amphipods and crayfishes. 
4. THE SWIFT-STREAM COMMUNITIES 
As the spring brooks and the intermittent streams continue to 
erode their beds, they increase the extent of their drainage systems and 
become larger streams. Springs tend to disappear in connection with 
the spring brook and the intermittent stream reaches the ground-water 
level and becomes permanent. The two sets of conditions converge 
