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HYDROGRAPHY.
■i 27
mum discharges, between maximum and mean discharges, and between mean
and minimum discharges. The discharges are given, partly in cubic meters per
second and partly in second-liters per square kilometer (si per sq. km) while
the mean run-offs are also given in millimeters.
The hydrographic conditions of the mountain rivers of Sweden derive their
characteristics from the high mountains, with the large precipitation. A
permanent covering of snow of a considerable depth remains, as a rule, for more than
half the year. In consequence, the spring-floods are of dominating importance,
while, at the same time, there exists a most marked and lengthy period of
low-water during the winter when only a small atmospheric precipitation is received
by the rivers, which are fed by the waters that are stored up in the lakes and
the subterranean supplies. The result of this is a great annual variation, which,
as a rule, is largest in the most northerly rivers. The absolute variation
amounts to between 4-^6 meters. The winter water-level falls considerably below
that of the summer, and this difference increases the farther north one goes.
The date of the lowest winter-level is retarded, too, towards the north.
The gradual melting of the snow, during the spring and the early part of the
summer, in the large river basins with their varying height conditions, has the
result that the spring flood is of long continuance and, as a rule, is divided
into several distinct floods. In the northern rivers we have the typical
occurrence of three, viz., "the home flood", occasioned by the melting of the snow
in the lower parts of the country; the "mountain flood", which is the result of
the melting of the snow at higher levels, and, finally, the "summer flood", to
which the melting of the snow in the very highest part of the mountains
contributes. In the southern rivers, again, it is, as a rule, only the first two of
these that occur, and these often unite to form one flood, which, in such cases,
often assumes very great dimensions. As a rule, the maxima of the spring floods
are higher the farther north one goes. As a rule, they occur at an earlier date
in the more southern rivers, although the volume of water stored by the lakes
can occasion variations in this respect, too.
By means of the numerous large lakes existing in, and immediately below,
the mountain regions, a large volume of water is stored up, serving to provide
a relatively large water-supply during the summer, so that the summer
low-water level is comparatively high. This height, as compared with the average
level, grows less and less, however, the farther south one goes.
The early period of the autumn at which the snow begins to lie permanently
and accumulate, has as a result that the autumn high-water levels are
inconsiderable, as a rule. As the figures in the table show, however, they become
more apparent the farther southwards one proceeds.
The discharges of the rivers is considerable, but, as a rule, it varies very
much during the course of the year. The head of exceptional high-water
is, thus, from 30 to 80 times as great as that of the corresponding low-water,
differences in this case depending more especially on the percentage of water
stored up by the lakes. (The figures, however, are somewhat unreliable, as, very
often, there exist no trustworthy measurements for high-water maxima.) The
mean run-off is, probably, greatest in the basin of the Luleälv, where it amounts
to 19 ’6 second-liters per square kilometer, which corresponds to a mean
water-depth of 620 millimeters on the total drainage-area. This fact stands in
connection with the great precipitation in the mountain regions of the basin
(vide p. 43). The run-off diminishes towards the south (the Umeälv = 540
millimeters), but rises again in the basin of the Indalsälven (590 millimeters),
where the high mountain district, in consequence of its vicinity to the sea,
has a greater precipitation. Thence they diminish considerably in the region
of the southern river-basins of Norrland (the River Ljusnan = 370 millimeters),
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