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As the submarine delta grows near to the level of the sea the distributaries of the river cover it with subaerial deposits altogether similar to those of the flood plain, of which indeed the subaerial delta is the prolongation.
Both owe their origin to a sudden check in the velocity of the river, compelling a deposit of the load; both are triangular in outline, the apex pointing upstream; and both are traversed by distributaries which build up all parts in turn. In a delta we may distinguish deposits of two distinct kinds, the submarine and the subaerial.
For about seven hundred miles from its mouth in Aral Lake the Oxus receives no tributaries, since even the larger branches of its system are lost in a network of distributaries and choked with desert sands before they reach their master stream.
Muddy shoals surround its front, shallow lakes, e.g. lakes Pontchartrain and Borgne, are formed between the growing delta and the old shore line, and elongate lakes and swamps are inclosed between the natural embankments of the distributaries. The delta of the Indus River, India, lies so low along shore that a broad tract of country is overflowed by the highest tides.
In the river provinces are many canals which act as distributaries during the spring overflow, carry the water a long distance and distribute it over a large area during the periods of inundation. In several places the usefulness of these canals has been increased by the construction of reservoirs which receive and hold the floods upon the plan proposed for some of our arid states.
The Mississippi has built a delta of twelve thousand three hundred square miles, and is pushing the natural embankments of its chief distributaries into the Gulf at a maximum rate of a mile in sixteen years.
As a river flows over its fan it commonly divides into a branchwork of shifting channels called DISTRIBUTARIES, since they lead off the water from the main stream. In this way each part of the fan is aggraded and its symmetric form is preserved.
It was constructed by the canal engineers while the ditch was being dug. There are 390 miles of branch canals from thirty to fifty feet wide and from six to eight feet deep, and 2,095 miles of distributaries, or ditches running between villages and squares.
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