along mud,
sand, and even boulders in their rapid course; the torrents in
turn deliver a large part of their loads to the river. As the rain
passes, the gulches become dry and remain so until another storm
visits the region. It is storming somewhere within the basin of
the Colorado much of the time, for the river drains two hundred
and twenty-three thousand square miles. So it comes about that
whether one visits the river in winter or summer one always finds
it loaded with mud.

But what becomes of all this mud? The river cannot drop it in the
narrow canons. It is not until the river has carried its load of
mud down to the region about its mouth, where the current becomes
sluggish, that the heavy brown burden can be discharged. Dip up
a glassful of the water near the mouth of the river, and let it
settle, then carefully remove the clear water and allow the sediment
in the bottom to dry. If the water in the glass was six inches
deep, there will finally remain in the bottom a mass of hardened
mud, which will vary in amount with the time of the year in which
the experiment is performed, but will average about one-fiftieth
of an inch in thickness. Each cubic foot of the water, then, must
contain nearly six cubic inches of solid sediment or silt.

It has been estimated that the average flow of the Colorado River
at Yuma throughout the year is eighteen thousand cubic feet of
water per second. From this fact we can calculate that there would
be deposited at the mouth of the river every year, enough sediment
to lie one foot deep over sixty-six square miles of territory.
Nearly one three-hundredth part of the Colorado River water is
silt, while in the case of the Mississippi the silt forms only one
part in twenty-nine hundred.

[Illustration: FIG. 3.--LOOKING TOWARD THE DELTA OF THE COLORADO
FROM YUMA]

Now we are prepared to understand the origin of the vast lowlands
about the head of the Gulf of California. Long ago this gulf extended
one hundred and fifty miles farther north than it does at present,
so that it reached nearly to the place where the little town of
Indio now stands in the northern end of the Colorado desert.

When the Colorado River first began to flow, it emptied its waters
into the gulf not far from the spot where Yuma is situated. The
water was probably loaded with silt then as it is now. Part of
this sediment was dropped at the mouth of the stream, while part
was spread by the currents over the botto