Flood plains and meandering streams are among the characteristics of maturity of stream valley development. Other characteristics are moderately sloping valley walls, a well-defined drainage system with many tributaries, and definite divides, a fairly well-established grade, and the absence of waterfalls and lakes.
The degree to which these characteristics are developed depends upon the part of the stage reached; that is, they are only partly developed in the stage of early maturity, but are fully developed in the stage of full or late maturity. The stage of early maturity is called the stage of adolescence.
Youth is a relatively rapid stage of valley development, maturity is longer, but old stage is of infinite duration. In the old age of a landscape, streams flow in wide meanders over an open plain.
Ox-bow lakes, the evidence of changing stream courses, because of lateral erosion and deposition, are common. The main portion of the land is at low elevation, and is relatively flat, with only a gentle slope in the direction of drainage.
Such an erosional plain is called peneplain. Sometimes the evenness of a peneplain is interrupted by an occasional hill.
Such hills or mountain remnants remain standing long after the surrounding land is reduced by erosion because hey are made of more resistant rock. Erosional remnants of this type are called monadnocks after the example of Mt. Monadnock in New Hampshire.
There are only a few streams and tributaries present to drain the flat, old-age landscape. The lack of drainage combined with the low gradients and pending of drainage behind natural levees can result in marshes and swamps.
However, few river valleys of today are in the stage of old age and most of them are in youth, or early maturity.
Before there has been sufficient time for their development beyond this stage, uplift or depression interferes with their development, and they start on a new cycle. This is called interruption in the development of a valley.
As a matter of fact, true peneplains do not exist, because all erosional plains have more relief features than the concept of the peneplains permits. The fact is that truly landscapes are aggradational.
However, there are many instances of erosion surfaces that are more or less dissected as a consequence of uplift.
When uplift of a land surface interrupts the erosional cycle at any point, vertical erosion towards base level starts afresh, and because of further deepening of valleys by erosion waterfalls and rapids are created. The landscape as well as its streams are then said to be rejuvenated.
As a result of new uplift during maturity or old age, after the creation of large stream meanders, those meanders become entrenched by the rapid incision of the rejuvenated stream.
Now, instead of eroding the land laterally, with meanders migrating across a plain, the rejuvenated streams erode vertically.
Incised meanders are created due to the fresh vertical down-cutting by the rejuvenated streams. Valley-in-valley sections are a common feature in mountainous regions. During the stable periods valleys may develop to maturity, but during the succeeding period of uplift they get rejuvenated.
The rejuvenation of mature valleys is caused by such factors as the sea-level changes, climatic changes or minor tectonic movements. The result is that the valley is slightly deepened, the old valley floor being preserved in strips along the edges.
The older valley floors are river terraces. However, some terraces are the result of successive periods of rejuvenation and aggradations.