Evolution
of Streams
The following characterstics can describe the evolution of
a single long-lived drainage system or the features of a single river from
the headwaters to the mid-section to the lowlands, especially if the headwaters
are in the mountains.
Early Stage Streams:
steep gradients down to a base-level created by a lake, larger river, or
ocean.
Vertical erosion dominates = V-shaped valleys with narrow floodplains
(Fig. 8.5).
Waterfalls or rapids are common.
The surrounding landscape shows high hills, deep valleys, and prominent
drainage divides.
With time, erosion does three things:
It cuts farther back into the hills (Headward Erosion);
It brings the elevation of most of the stream channel nearer to base level;
It reduces the elevation and relief of the surrounding landscape.
Middle-Stage Streams:
characterized by longer drainage systems with more tributaries, moderate
gradients, and increasingly rare waterfalls and rapids.
The lower gradients reduce the importance of vertical erosion while
increasing the importance of lateral erosion. This creates meandering channels,
wider floodplains, and broad, flat valleys within otherwise hilly landscapes.
With more erosion Middle-Stage Streams become Late-Stage Streams.
Late-Stage Streams:
these have low gradients and extensively developed meander belts characterized
by wide floodplains and extensive systems of ox-bow lakes, back swamps, and
yazoo streams.
The overall landscape is fairly flat.
It is unlikely that one stream will go through all
stages of stream development because over the lifetime of a stream, climate
may change, there is tectonic upheaval along the stream course, sea level
drops/rises, and/or the nature of the bedrock changes as the stream erodes
downward.