Figure 2-4. Erosional features of fluvial systems
Youth. A youthful stream is one that is undergoing initial development; consequently, it has
a steep, irregular gradient, few tributaries or meanders, and a narrow floodplain. In addition, most of the erosion
takes place in a downward direction. Figure 2-5 illustrates the topography of an area containing a youthful
Maturity. As time progress and erosion continues, the stream gradient decreases. The
number of course deflections, on the other hand, increases so that meanders begin to firm. Eventually, the degree
of lateral erosion becomes equivalent to that of downward erosion. This intermediate stage of development is
referred to as maturity. Figure 2-6 shows an example of a mature stream.
Old Age. The final stage of stream evolution occurs when lateral erosion takes precedence
over downcutting. These so-called old-age streams have very low gradient and wide floodplains. Table 2-1, page
2-12, lists the characteristics of each of the stages of stream evolution.
Occasionally, the land mass underlying an old-age stream is uplifted, and the stream is rejuvenated so that the
stages are repeated in a cyclic manner. Therefore, the three stages of development are collectively referred to as
the fluvial cycle of erosion.
(2) Depositional Features of Running Water. There are several factors that may cause the velocity of a
sediment-laden stream or river to decrease. For example, the gradient may decrease, floodwaters may subside, or
the water may evaporate or soak into underlying porous materials. In any case, as the velocity decreases, the
sediments are deposited. The heaviest and coarsest materials are deposited first, while the lightest and finest
particles remain in suspension for a longer period of time, traveling a greater distance from the source. The term
alluvium is applied to all fluvial deposits with the exception of deltas, which occur in lakes or seas, and
glaciofluvial deposits, which are generally referred to as outwash deposits.