In the stage of maturity waterfalls and rapids of youth disappear and they are replaced by a smoother stream profile. As the strong vertical erosion ceases, the stream develops meanders which cut back the interlocking spurs of the youthful valleys.
Now, the lateral erosion and stream deposition increase widening the valley floor. Spurs, at this stage, are partly buried beneath the floodplain deposits.
Barring a few exceptions in mountain drainage the levels of main and tributary valleys are accordant. This is so because despite the smaller volume of the tributaries, steeper gradient of the tributaries enable them to keep place with their master streams in down cutting.
It is worthwhile to remember that in the use of the terms youth and maturity, the idea of stage rather than age in years is conveyed. Varying factors like height, nature of rock and volume of river, introduce elements of variation in the rate of valley development.
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It is more common that different sections of a river system are in different stages of the cycle of erosion. It is not correct to say that a large river is young or mature, though it is easier to classify any given part of it.
Since the main streams and their tributaries are graded at this stage, they start forming flood plains.
The divides/interfluves become narrower, sharper and ridge-like, and the slopes along the valley sides decline. The relief shows progressive decrease as the summits are lowered more rapidly.