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Gary Minsky, age 14, of Pine Falls, Manitoba, Canada, for his question:

How do river deltas form?

It seems odd that a rolling river chokes its mouth with soggy silt. In time, it dumps muddy deposits that force the main stream to fan out in numerous separate channels. All large rivers and most smaller ones form these wide mouthed deltas.

Hence, it seems logical to look for some basic reason in the nature of things. The hows and.whys that govern the formation of deltas are in the natural behavior of earth and water .

When a mighty river meets the mightier ocean, we can expect great things to happen The force of. the rolling current suddenly collides with the more forceful wall of  resisting water. The river's energy is pressure from its volume of streaming water plug the speed it gained from flowing down slope. Its moving waters dissolved and eroded mud, roll in its path and its energy swept them along to the seat..

The force of a great river is enormous and when it reaches its rendezvous with  the sea .it carries with it countless tons of earthy solids. Fine fragments of clays ire suspended is the water ... .Gritty gravels, sands and heavier pebbles are swept under the stream bed. All these earthy fragments are pushed along by the energy of the rolling river.

Them suddenly the river confronts the stronger resistance of the ocean. . The out going tides and the gentle slope of the submerged continental shelf work to pull the river current out to sea. But the resisting wall of ocean water outranks them. It. forces the river to slow down and lose some of its energy. It can no longer carry its load of earthy materials and they sink to the bottom. Salty chemicals in the sea water tend to coagulate the finer suspended fragments and these heavier particles speed up the sinking process.

Gradually the deposits clog the mouth of tile river, though tidal action draws vast quantities out to sea.. Meantime, the choked river is pushed by  flowing water from behind. The main channel is blocked, but the pushing water'euts many smaller, shallow channels through the sogg y.mud.    As these channels become clogged, new ones are cut.  . The river mouth becomes a wide, fan shaped delta of soggy, muddy deposits filtered with a network o€ shifting waterways. It forms and builds gradually, always changing. The greatest  changes occur during floods when forger volumes of water carry more mud to ,be dumped i1r the delta..

The area of a major delta is enormous and from time to time the choked river must change its course. The mighty Mississippi delta covers 12,000 square miles near the shore. Its undersea deposits in the Gulf of Mexico are even larger and drillings indicate them to be 30,000 feet thick. Several times during the past 3,000 years, the great river has abandoned its old delta and cut a new course to the sea.

 

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