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Bill Banker, age 10, of Oakland, Calif., for his question:

What forms an alluvial fan?

An alluvial fan is first cousin to a delta. Both are spreads of muddy debris dropped by running streams. 11 delta is a flat triangle of muddy silt dropped where a river meets the sea. An alluvial fan forms where a mountain stream runs down into a valley. The wide end of the fan spreads over the valley floor. The narrow end is tipped up the slope.

The word alluvial refers to mud, silt, sand and gravel deposited by water. The engineer who makes the alluvial fan is a mountain stream gushing its way through a steep sided gorge. It has speed and power enough to tote along quantities of muddy debris. New debris is always being added as the stream dashes against its stony prison walls, shipping off fragments and loosening pebbles,

Suddenly the gorge widens and the busy little stream runs dawn onto a wide flat valley floor. No longer pushing its way through the gorge, the stream sud0enly looses much of its energy. It slows down and when this happens the heavier particles toted in the water begin to sink, They fall to the bottom creating a floor of sediment,. The stream must now cut its wry through this mud and it does so by dividing into channels which link and separate. It becomes what is called a braided stream.

From time to time, the head waters of the stream bring floods, the braided stream is swamped and new channels are cut in the alluvial deposit. These new channels seek the lowest areas in the mud and the fan builds up a symmetrical shape with one deposit upon another.

Upstream in the valley, the edge of the fun is being carried away by erosion. Meantime the stcep slope down from the mountains is built into a gentle curve. Instead of coming down into the valley with one big jump, the stream now slides down with a smooth swoop, The alluvial fan is quite an engineering feat and. it is  worked out by the stream to get an equal balance of forces between its  stoop sloping, gorge and the flat wide valley floor.

Though the same in character, each alluvial fan has its own features,

A wide stream with a load of fine debris tends to build a wide., gently sloping fan. A small stream, carrying a coarse load of sand end gravel, tends to build up a short, steep fan.

In most cases, the alluvial fan is not a long lived feature on the face of the earth. If for some reason the stream which made it dries up, it will last longer but even so, wind and weather will finally wear it away. Once in a while, an alluvial fan spreads clear across a valley floor, When this happens, a natural basin is formed. Into this hollow may be trapped the waters of the stream which made the fan together with the waters of other mountain streams, The hollow basin caused by this type of alluvial fan may in time become a lake.

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