. Elementary physical geography;. of the old one form these there may be several. Flood-plains and terraces are, therefore, incidents in thehistory of a river. Perhaps most of the rivers of the UnitedStates are in the flood-plain stage of their existence. Someof the streams of the northeastern part arc in the terracestage and arc approaching the period of old age. THE WASTING OF THE LAND: RIVERS 121 Deltas.—Salt water has a remarkable effect in clearingmuddy, fresh water, and the moment the two mix, thesediment held in suspension is deposited. Unless the sedi-ment is swept away by
. Elementary physical geography;. of the old one form these there may be several. Flood-plains and terraces are, therefore, incidents in thehistory of a river. Perhaps most of the rivers of the UnitedStates are in the flood-plain stage of their existence. Someof the streams of the northeastern part arc in the terracestage and arc approaching the period of old age. THE WASTING OF THE LAND: RIVERS 121 Deltas.—Salt water has a remarkable effect in clearingmuddy, fresh water, and the moment the two mix, thesediment held in suspension is deposited. Unless the sedi-ment is swept away by currents and tides, a considerableaccumulation will form at the mouth of the river. The mouth of the Mississippi River shows an interestingtype of delta formation. In this case it is evident that thebanks of the delta are self-made, and they have been formedbecause the current has been checked more effectually atthe edges than in mid-stream: Since the lower Mississippihas occupied its present channel, the river has extended its. TERRACES IN A FLOOD-PLAIN Each marks a stage of down-cutting. The darker shading shows the old bed of the river. lower part about one hundred miles into the Gulf of Mexico. The deltas of the Volga, and Ganges-Brahmaputra areolder and more complex than that of the Mississippi. Theyare also more compactly filled with sediment. The deltaof the Ganges-Brahmaputra is perhaps the most extensiveknown. Its frontage on the Indian Ocean is about twohundred miles, and its area is greater than that of of the land consists of shifting mud-flats, and thewhole region is subject to destructive inundations. The delta of the Adige-Po has developed in a manner notunlike that of the Ganges. Probably no other river of its 122 PHYSICAL GEOGRAPHY size brings down more sediment than the Po. As a result,its delta isjSlling and extending so rapidly that the town ofAdria, in Julius Cicsars time a seaport, is now more thantwenty miles inland. Ostia, once at the mouth
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