The development of stream valleys takes place in a orderly fashion. A valley passes through three stages during its evolution. The three stages are (i) Youth Stage, (ii) Mature Stage, and (iii) Old Stage.
(1). Youth or Early Stage:
A stream is said to be in the youth stage when it cuts its valley downward to establish graded condition with its base level.
- Position: The youth stage is commonly found in mountainous regins from where a stream starts its journey.
- Erosion: Down-cutting is dominant.
- Valley: Narrow V-shaped valley.
- Longitudinal Profile: Longitudinal profile is ungraded. The gradient is steep and water falls and rapid are common.
- Valley Floor: The stream occupies most of the width of the valley floor as a result there is little or no flood plain.
- Stream Pattern: The stream course is angular and without meanders. Tributaries are short and few.
(2). Mature Stage:
A stream is said to be in a mature stage when downward erosion diminishes and lateral erosion dominates.
- Position: Mature stage is found in the plains lying adjacent to the mountain region.
- Erosion: Downcutting is slight and sidecutting becomes dominant.
- Valley: Broad and trough shaped.
- Longitudinal Floor: The stream swings in meanders. The flood plains are narrow and sandbars are present.
- Stream Pattern: The stream moves in meanders. The tributaries are many.
(3). Old Stages:
In the old stage of valley development, the flood plan of a stream becomes several times wider then its meander belt.
- Position: The old stage is fond near the mouth of streams.
- Erosion: In the old stage, the stream ceases to enlarge the flood plains. The man work of a stream is to rework the unconsolidated sediment of the flood plain.
- Valley: Valleys become wide and open with low boundaries which may be indistinct.
- Longitudinal Profile: The gradient become very lo. The stream approaches base level it aggrades strongly.
- valley Floor: Ox Bow lakes are very common. Natural levees are very are also present, which are accompanies by back swamps and yazoo tributaries. The meander belt is narrower than the valley floor.
- Stream Pattern: The stream pattern is meandering with oxbow lakes. The tributaries are few and large.
From the above discussion it is assumed that the base level of a stream remains constant as a river progresses from youth to old stage. On many occasions, however, the land is uplifted. The effect of uplifting on a mature stream is to abandon lateral erosion and revert to downcutting. Such rivers are said to be “rejuvenated”. Mature streams readjust to uplift by cutting a new flood plain at a level below the old one. This produces step-like features in the river valley which are called “river terraces”.