A £reai river is broad and deep at its jnoutk, and

its current is very slow; but it carries sediment even

to the ocean.
We have been describing a large river that had ita aource in the

mountains; but others are much smaller, and man; do not start in

the raountaiiiB. Some empty their water into other rivers, being

tributaries, and others enter lakes rather than the sea. They may

also have low,

sof t ba nks i nstead

of high, rocky

ones, and there

may be neither

rapids nor falls.
But no matter

where their

sources and

mouths may be,

or what other dif-

erences may ex-
ist, they are, in Via. 42.
many ways, much A pebblj brook bed which is filled with water when

like thifi river. '-''^ '^'>''' '^^s or tbe aaow melts, but is often dry in

summer-
Where does 80 much water come from T Taken up

from the ocean, it falls from the sky in the form of rain



jvGoot^lc



44 BOMB GEOORAPBT

or BDow. But we all know that small Btreams dry up
and disappear soon after a rain. Even large brooks may
become quite dry in summer (Fig. 42). Why, then, do
not great rivers also dry up ?

One reason is that many rivers have a constant supply
at their source. That this is true of a stream starting in
a high mountain is clear, because we have seen (p. 26)
that the snow ia such places never entirely melts away.
It is also true of streams that have their sources in lakes
and swamps.
