vu RIVERS AND LAKES 333 
Some rivers, on the other hand, offer no 
such periodical differences. The lower Rhone, 
for instance, below the junction with the 
SaOne, is nearly equal all through the year, 
and yet we know that the upper portion is 
greatly derived from the melting of the Swiss 
snows. In this case, however, while the 
Rhone itself is on this account highest in 
summer and lowest in winter, the Sadne, on 
the contrary, is swollen by the winter’s rain, 
and falls during the fine weather of summer. 
Hence the two tend to counterbalance one 
another. 
Periodical differences are of course com- 
paratively easy to deal with. It is very dif- 
ferent with floods due to irregular rainfall. 
Here also, however, the mere quantity of rain 
is by no means the only matter to be con- 
sidered. For instance a heavy rain in the 
watershed of the Seme, unless very prolonged, 
causes less difference in the flow of the river, 
say at Paris, than might at first have been 
expected, because the height of the flood in “ 
the nearer affluents has passed down the river 
before that from the more distant streams has 
