

" Parr IL Secor. ii.§3.] SOURCES OF RIVERS. 359. 
contents are derived partly from rain (including mist and dew) and 
melted snow, partly from springs. In a vast river system like that 
_ of the Mississippi, where the area of drainage is so extensive as to 
embrace different climates and varieties of rainfall, the amount of 
discharge, being in a great measure independent of local influences 
of weather, remains tolerably uniform or is subject to regular 
periodically recurrent variations. In smaller rivers, such as those 
of Britain, whose basins lie in a region having the same general 
features of climate, the quantity of water is regulated by the local 
rainfall. A wet season swells the streams, a dry one diminishes 
them. Hence, in estimating and comparing the geological work 
done by different rivers, we must take into account whether or not 
the sources of supply are liable to occasional great augmentation or 
diminution. In some rivers there is a more or less regularly 
recurring season of flood followed by one of drought. The Nile, fed 
by the spring rains of Abyssinia, floods the plains of Egypt every 
summer, rising in Upper Egypt from 30 to 35 feet, at Cairo 23 to 
24 feet, and in the seaward part of the delta about 4 feet. The 
Ganges and its adjuncts begin to rise every April, and continue 
doing so until the plains are converted into a vast lake 32 feet 
deep. In other rivers sudden and heavy rains occurring at 
irregular intervals swell the usual volume of water and give rise to 
floods, freshets or “spates.” This is markedly the case with the 
rivers of Western Europe. Thus the Rhone rises 114 feet at Lyons 
and 23 feet at Avignon ; the Sadne from 20 to 244 feet. In the middle 
of March 1876, the Seine rose 20 feet at Paris, the Oise 17 feet near 
Compiegne, the Marne 14 feet at Damery. The Ardéche at 
Gournier exceeded a rise of 69 feet during the inundations of 1827. 
The causes of floods, not only as regards meteorological conditions, 
but in respect to the geological structure of the ground in which: 
_ the floods are produced, merit the careful attention of the geological 
student. He may occasionally observe that, other things being equal, 
the volume of a flood is less in proportion to the permeability of a 
_ hydrographic basin and the consequent ease with which rain can 
sink beneath the surface. 
Were rivers entirely dependent upon direct supplies of rain, they 
would only flow in rainy seasons and disappear in drought. This 
does not happen, because they derive much of their water not 
directly from rain, but indirectly through the intermediate agency of 
springs. Hence they continue to flow even in very dry weather, 
because, though the superficial supplies have been exhausted, the 
underground sources still continue available. In a long drought, 
however, the latter begin to fail, the surface springs ceasing first, and 
eradually drying up in their order of depth, until at last only deep- 
seated springs furnish a perhaps daily diminishing quantity of water. 
Though it is a matter of great economic as well as scientific interest 
to know how long any river would continue to yield a certain 
amount of water during a prolonged drought, no rule seems 
