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In the valley tract (middle course or maturity), a river or stream erodes more laterally than downwardly. Although this region contains significant deposit of fluvially transported material, much of the deposited material is eroded and transported many times during the different floods. During maturity, the stream divides become narrower as the valley floor broadens and the stream begins to meander. The width of the valley is usually 10-15 times that of the channel. The erosion is often in bedrock or a shallow covering of sediment unless downwarping of the earth's crust has caused significant aggradation.
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