258 University of California Publications in Zoology [Vol. 27 
riffles, and often by summer (July and August) all but the very 
largest and deepest pools disappear. 
To illustrate graphically the seasonal cycle of a foothill stream, 
recourse has been had to the records of stream measurements which 
have been made during the past two decades by the United States 
Geological Survey. The amount of water passing in many of the 
streams where measurements have been made is of course greater than 
in the small creeks inhabited by the bulk of the boylii population. 
Yet the seasonal history of the larger and smaller streams is probably 
much the same save perhaps for the midsummer dry or low-water 
period. 
The stream chosen for study is Arroyo Seco Creek on the east slope 
of the Santa Lucia Mountains in Monterey County. This creek, 
although of considerable size, is known to be inhabited by Rana boylii 
boylii (Storer, 1923, p. 8), the record of water measurements for the 
creek is complete over a considerable period, and the stream is 
unmodified by man either by diversions or dams at, or above, the place 
at which the measurements here used were taken. The data used 
therefore give the normal behavior of such a creek under the variations 
to be met with in a ten-year seasonal cycle of climatic conditions in 
California. The drainage basin of this stream is moderate in extent 
(215 square miles). Arroyo Seco Creek heads in the Santa Lucia 
Mountains where a small amount of snow falls during the winter 
months. The lower portion of the drainage basin lies in the foothill- 
chaparral region. 
The complete and principal graph (text fig. PP) shows the average 
daily discharge of water in second-feet during the decade, 1909-1918. 
The two shorter graphs show, respectively, the maximum and minimum 
discharges during the season in which the spawning and larval devel- 
opment in boylii is carried on. The record of daily discharge in 
second-feet was chosen rather than the gage-height because the former 
exhibits more strikingly the violent changes in amount of run-off 
which follow precipitation in the drainage basin. For convenience in 
plotting, only the readings on the first, seventh, thirteenth, nineteenth, 
and twenty-fifth days of each month were used. The great extent to 
which such a stream may be swollen as a result of heavy rains over 
the drainage area is indicated by the maximum discharges recorded 
for certain dates in the years included in the record. These are as 
follows : 
