The Geographical Journal (Journal of the Royal Geographical Society): Volume X, No. 6 [‎115r] (82/186)

POTAMOLOGY AS A BRANCH OF PHYSICAL GEOGRAPHY.
621
variations of the quantity of water than by variation of the heights of a river. Not
all rivers are constantly flowing; a great many of them dry up in dry seasons or
dry years. It is very hard to separate those two types on maps, in order to gain
true conceptions of the water-supply of different countries. Those rivers which
are covered with ice during the winter, as all great rivers of higher latitudes are,
deserve special interest. Mr. Rykatscheff has arrived at important conclusions
as to climatology based on the time of the closing of rivers by ice, and it needs no
words to say how great are the practical advantages of an exact knowledge of the
length of time during which our great waterways are ice-bound. Dr. Forster has
shown, in his discussion on river-temperatures, how one can get a forecast of ice-
formation by an observation of the river-temperatures, and he thus proved the
necessity of measuring the river-temperatures, which, as we have already seen, play
an important part in the movement of the water. Numerous rivers lose their
water in limestone districts showing Karst phenomena. They have for several
years attracted interest, especially in Austria. Other rivers lose their water in
their own gravels, and the number of such blind-ending rivers is rather great.
Dealing with the action of w r ater on its bed, potamology comes in close contact
with geology and geomorphology. It is now a nearly generally accepted theory
that valleys have been dug out by rivers, and may be regarded as widened river-
courses. Prof. W. M. Davis has worked out the general system of evolution of
river-basins, but the action of rivers on their beds cannot yet be traced out in all
directions. There seems to be a correlation between the quantity of water, the width
and depth of a river, hut which is the normal relation between the two quantities is
not determined, and there has been established no delimitation between deep and
shallow rivers, the former deepening their courses, the latter shifting from side to
side. The conditions under which the river begins to wind, forming meanders, are not
cleared up. Even the way it pushes its detritus forward is not sufficiently known.
Very flat mountain rivers permit us sometimes to recognize how small stones are
transported. They are rolled and pushed forward considerably slower than the
water itself runs ; with increasing velocity, the whole bott>m of such a river begins
to move, and this is also the case with larger rivers. When they are in flood, then
all the bottom gravel is moving; whilst in general every pebble goes separately on
its way from one gravel bank to the next. Thus the gravel banks move downward.
The changes of the river-bed produced in this way can only be stated by comparing
exact maps of the river bottom made at different times. Maps of the rivers, in a
very large scale, with indications of the depth, are as necessary as bathymetrical
maps of lakes. The maps of some river-mouths made for nautical purposes, and
the excellent maps of the rivers of Holland made by the Water-statt, may be
regarded as very good examples. It must be hoped that they will be imitated
elsewhere. The fine river descriptions which have been published in Germany show
in general a want of maps of the river-beds.
No less attention than to the bottom material of the rivers must be attached to
the suspended material they contain. It is the waste of land carried into the sea.
It affords the means, as Dr. J. Croll and Sir Archibald Geikie have shown, for
determining the duration of geological time, and considerable scientific interest
must therefore be attached to its evaluation. The river Nile shows how great the
practical value of river mnd is; Egypt owes its existence to the regular inun
dations and refreshing of the soil by the Nile. Careful determinations of the quanti
ties of river mud have been undertaken here and there merely for economical
purposes. Finally, the materials dissolved in the river water require attention and
examination. Rivers coming from old gneissic and granitic rocks have everywhere
a dark colour, owing to dissolved humic alkaline salts. These are the black rivers