The Geographical Journal (Journal of the Royal Geographical Society): Volume X, No. 6 [‎114r] (80/186)

( 619 )
POTAMOLOGY AS A BRANCH OF PHYSICAL GEOGRAPHY.*
By Professor ALBRECHT PENCK, Ph.D.
Of the different departments of physical geography treating of the hydrosphere,
none has advanced more slowly than the science of rivers. Oceanography has
developed in a wonderful way. The limnology advocated by Forel at the London
Congress of 1895 has become a separate flourishing branch; only the hydrology of
running water is still in a very unsatisfactory state. The fact that it has not a
name of its own corresponding to oceanography or to limnology, indicates its
neglected position, but there can be no doubt that it must gain equal rank and
follow the same evolution as the two other above-named branches of hydrology.
The science of rivers, which may be called potamology, must be treated under
five different heads—
1. The physics of running water.
2. The volume of water and its fluctuations.
3. The action of water on its bed.
4. The distribution of running water on the Earth.
5. Rivers as a scene of organic life.
As to the physics of running water, hydrotechnologists have recognized the
dependence of velocity on the declivity of the water surface and depth. But all
formulas based on these elements alone are not satisfactory. Thus neither the
old formula of Chezy, the newer of Bazin and Darcourt, nor the complicated one
of Ganguillet and Kutter can be accepted. The problem is, in fact, more complex
than has been generally supposed. If we have a body of moving water, its motion
will depend—
(i.) Upon its own mobility, changing with its temperature.
(ii.) Upon the forces of gravity and rotation acting upon it.
(iii.) On the resistance caused by the form of its bed and by its surface.
The mean velocity of a river may vary—
(а) With its temperature.
(б) With its volume, height of the fall, and geographical latitude.
(c) The width and declivity of the river bottom and the area of the river
surface.
Some physicists have commenced to treat the question in a theoretical way, and
have shown how complicated the movement of each water-particle is—how it moves
in very intricate spirals. But a more empiric way can be followed also, by
establishing relations between the factors just named and the results of stream-
measurements. We must not forget that the only way of determining the mean
velocity of a stream is by measuring the volume of water which passes through a
given channel in a given time.
In treating of the volume of water, potamology comes into intimate relation
with climatology. All river water (with the very inconsiderable exception of the
water which comes from volcanic vents) is derived from the sea. No part of the
water which is evaporated from the sea and has fallen as precipitation on the land
is fixed there, for we see no general sinking of the sea-level, which should happen
if any measurable part of the rainfall were permanently held by the soil. The
quantity of river water is therefore equal to the quantity of sea water which is
carried as moisture in the air to the land. The first rough estimation of the
quantity of river water pouring into the sea by Dr. Murray shows that it is
* Paper read in Section E (Geography) at the Toronto Meeting of the British
Association, August, 1897.