Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 75

• Climate
• Vegetation
• Topography
• Rainfall
• Geology

Rivers and streams go through an evolutionary process as they age and follow the general
sequence depicted in Figure 3.11a through d (Dutch 2009).
A young stream is characterized by a V-shaped valley, rapids and waterfalls, and no
floodplain (Figure 3.11a). Over time, a young stream deepens its valley (Figure 3.11b). As
the stream matures, it begins to develop a floodplain and sand and gravel bars. The valley
deepening slows (Figure 3.11c). A fully mature stream exhibits a very wide floodplain with
pronounced meanders and cut-off meanders called ox-bow lakes (Figure 3.11d) Young
streams can also be characterized as predominantly eroding streams and fully mature
streams as depositional streams (Leopold 1992).
The drainage development pattern of a watershed may yield some clues about its sub-
surface geology. For instance, Figure 3.12 shows a dendritic drainage pattern with a higher
drainage density in the headwater region than in the depositional zone. In the headwater
region, silty clay underlies the streams; whereas in the depositional zone, the sediments
are porous with a high-infiltration capacity. Since the clay sediments in the headwater
region are not very permeable and restrict infiltration, most of the precipitation drains as
surface water. The result is a denser stream drainage pattern required to drain the same
amount of land. This same drainage pattern can also be formed by a steeper slope gradient
in the headwater region than in the depositional zone.
There are other processes at work that determine a watershed's drainage density, so we
must consider other factors in addition to soil infiltration capacity. For example, ground-
water at the surface shapes the Earth's topography through a process known as seepage

(b)

(a)

(c)

(d)

FIGURE 3.11
(a) Young stream, (b) mature (early) stream, (c) mature (late) stream, and (d) old age stream. (From Dutch, S., Erosion
and landscape evolution, University of Wisconsin Green Bay, http://www.uwgb.edu/dutchs/EarthSC202Notes/
erosion.htm, 2009. With permission.)