THE INDIAN PENINSULA 133

Some of the upper affluents of the Indus have been
already mentioned in connection with Afghanistan and
Kashmir, so that west of the Indus we need but consider
those western tributaries which are derived from the
basins of the Kabul, the Kuram, the Tochi, and the
Gomal. These are all closely connected with the military
policy of our frontier ; all of them afford means of communication
 between India and the trans-border districts,
and are much more important to us as military highways
than as channels of commerce with Central Asia. As they
drain the slopes of mountainous regions they run in fixed
channels, passing over rocky beds, often as mountain
torrents, and always with a rapid and dangerous current
when under the influence of floods caused by melting
snow or by heavy rain in the hills about them. On
emerging into the open plain of the Derajat, the Kuram,
Tochi, and Gomal pass through beds of alluvial soil which
tends to a constant shifting of their channels ; but as the
distance from their debouchment into the plains to their
junction with the Indus is short, such changes are
comparatively unimportant. The Kabul River and its
mountain-bred tributaries, the Kunar and the Swat, also
flow through well-fixed channels with little apparent
variation from century to century. But when we examine
the Indus and its great Punjab branches, we find that
in their character as rivers of the plains they evince a
constant tendency to shift their positions from one bed to
another. So long as they are rivers of the hills they obey
the same laws as are common to all hill streams. The
spurs of the mountains hold them in their rocky channels,
and not even the influence of vast landslips, blocking their
way and forming deep reservoirs, has any serious disturbing
 effect on the course they run. Under favourable conditions
 they change their grade, filling up the valleys with
detritus ; and, cutting back at their sources, capture fresh
areas from the highlands. It not infrequently happens,
indeed, that the entire flow of the Indus is held up by a
mighty barrier of fallen rocks and débris, and the narrow
channel above the fall rapidly deepens to a reservoir from