Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 421

Dendritic Parallel Rectangular

(a) (c)

(b)

FIGURE 12.3
Drainage patterns. (a) Dendritic, (b) parallel, (c) rectangular.

Here, the subsurface geology has a similar resistance to weathering, and so there is no
apparent control over the direction the tributaries take. Parallel drainage patterns (Figure
12.3b) form where there is a pronounced slope to the surface. A parallel pattern also devel-
ops in regions of parallel, elongate landforms where there are outcropping resistant rock
bands. Tributary streams tend to stretch out in a parallel-like fashion following the slope
of the surface. A parallel pattern may also indicate the presence of a major fault cutting
across an area of steeply folded bedrock. The rectangular drainage pattern (Figure 12.3c) is
found in regions that have undergone faulting. Streams follow the path of least resistance
and become concentrated in places where exposed rock is the weakest. Movement of the
surface due to faulting offsets the direction of the stream, causing the tributary streams to
make shape bends and enter the main stream at high angles (Ritter et al. 2002).
In the subsurface zone of watersheds, aquifers lend form and structure to the land sur-
face above. Different bodies of water such as lakes and wetlands are the representation of
exposed groundwater in humid environments. In terms of structure, if too much water is
pumped from an aquifer, the land above it will subside. Land subsidence has occurred in
urban areas of Texas, California, Colorado, Delaware, and Louisiana (USGS 2000).

12.2.3 Zonation of Surface Watersheds

Watersheds are organized as a nested hierarchy, having smaller basins set inside the next
larger basin. The Missouri, Ohio, and Arkansas basins, for example, are nested within the
Mississippi watershed. Stream discharge increases as each basin adds its runoff to the
next. As a result, the average discharge of most watersheds increases with total drainage
area (Figure 12.4).
There is also a hierarchy to the stream channels within a drainage network. Streams
occupying a drainage basin form a hierarchical network of channels that, in humid areas,
hold increasingly larger volumes of water as one moves toward the mouth of the basin. A
stream's order is its rank, or relative position, within the network (Strahler 1952). A first-
order stream is a channel with no tributaries, and channels of this type occur primarily at
the higher elevations (headwaters) of the watershed. A second-order stream is a channel
fed by at least two first-order tributaries. The joining of two-second order streams forms
a third-order stream. Ranking continues until the highest-ordered channel is reached
(Figure 12.5). First and second-order streams are located in the headwaters and typically
convey small volumes of water.