Now in all this rocky and
denuded region the water that falls runs immediately to the river, and
causes terrible freshets therein; so operations capable of keeping the bed
dry would be out of proportion to the probable results of the
exploitation, whence it follows that the latter is only possible in dry
weather, and these deposits are therefore called "dry season washings."
These deposits are still worked in our day as they were in the time of the
Portuguese. In order to dry the bed a dam is constructed, and the river is
either diverted into a plank flume supported by piles, or into a canal dug
along the shore, or by means of tight walls, according to the lay of the
place. The second process, which is preferable to the first, is in fact
impossible when the river runs, as is often the case, in a narrow, abrupt,
walled channel. These works are sometimes very important. In 1881, the
Acaba Mundo flume was 140 meters in length and 5.2 m. wide, and, with a
velocity of 2.25 m., discharged 4,500 liters per second; still longer ones
might be cited that discharged as much as 8,000 liters.
In the dry part of the river the extraction of the sand, stones, and
cascalho is done solely by hand.
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