32 Physical Geography of the Amazons Valley. |January, 
region which I have called a lower terrace. As soon as they 
enter this region the rapids cease, and immense flood-plains 
spread out on both sides. Besides the fact that these flood-plains 
are out of all proportion to the rivers, they are remarkable for 
the extreme irregularity of their borders, Every little stream 
which enters the main affluent passes through a flood-plain of its 
own, often five or six miles broad, though the stream itself may 
be hardly as many feet across. Crooked bays of varzea extend 
far into the mainland; numberless islands of ¢erre-firme are scat- 
tered over the flood-plain; and the most conscientious map-maker 
who attempts to unravel this tangle is likely to give up in despair. 
The irregularity generally increases toward the. Amazons, where 
the alluvial land of the tributaries spread out broadly until it is 
lost in the Amazon flood-plain. 
Three great tributaries—the Tapajós, Xingu and Tocantins— 
flow down over the southern table-land from the center of South 
America. Geographically it should be said, the Tocantins cannot 
be regarded as a tributary of the Amazons; its mouth, called the 
Para river, receives a portion of Amazonian water, but it opens 
into the Atlantic and is separated from the main mouth of the 
Amazons by Marajó. Physically the three rivers resemble each 
other closely. They are all clear-water streams, flowing down, with 
many rapids, to a point about 150 miles from their mouths, where 
the rapids cease, and the rivers gradually expand into quiet lakes. 
The lakes are bordered by bluffs, edges of the table-land and 
continuous with those which border the southern side of the 
Amazonian flood-plain. In their lower portion these lakes are 
from seven to ten miles wide and very deep; they have hardly 
any current, but rise and fall with the tides as regularly as the 
sea. At their northern ends they are suddenly contracted by the 
Amazonian flood-plain, and here they receive Amazonian water 
through narrow channels or furos. The furos, where they open 
into the lakes, are still bringing in Amazonian sediment, and they 
have thus pushed their mouths far into the clear water. The Tapa- 
jos and Xingu finally reach the Amazons through embouchures 
less than half a mile broad—about the average width of these 
rivers near their lower falls. 
Some smaller rivers which enter the Amazons from the south 
have more or less muddy waters, and these have filled up their 
valleys with sediment. The flood-plains thus formed are bordered 
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