Deltas

When a stream enters the relatively still water of a lake or the ocean, its velocity and capacity to hold sediment drop suddenly. Thus the stream dumps its sediment load here, and the resulting deposit is known as a delta. Where a coarse sediment load of an alluvial fan dumps its load in a delta, the deposit is known as a fan-delta. Braid-deltas are formed when braided streams meet local base level and deposit their coarse-grained load. When a stream deposits its load in a delta, it first drops the coarsest material, then progressively finer material farther out, forming a distinctive sedimentary deposit. The resulting fore-set layer is thus graded from coarse nearshore to fine offshore. The bottomset layer consists of the finest material, deposited far out. As this material continues to build outward, the stream must extend its length and forms new deposits, known as topset layers, on top of all this. most of the world's large rivers—the

Mississippi, Nile, and Ganges—have built huge deltas at their mouths, yet all of these differ in detail.

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