Download The Drainage Basin Concept

The Drainage Basin Concept
Geomorphologists and hydrologists often view streams as being part of
drainage basins. A drainage basin is the topographic region from which
a stream receives runoff, throughflow, and groundwater flow.
Drainage basins are divided from each other by topographic barriers
called a watershed (Figure 10aa-1). A watershed represents all of the
stream tributaries that flow to some location along the stream channel.
The number, size, and shape of the drainage basins found in an area
varies with the scale of examination. Drainage basins are arbitrarily
defined based on the topographic information available on a map. The
quality of this information decreases as map scale becomes smaller (see
topic 2a).
Figure 10aa-1: The following image shows the nested nature
of drainage basins as determined from a topographic map sheet.
The red lines describe the watersheds for the drainage basins of
first order streams. The yellow lines define the watersheds for
two drainage basins from locations further upstream. Note that
the first order basins are components of these much large
drainage basins.
Drainage basins are commonly viewed by scientists as being open
systems. Inputs to these systems include precipitation, snow melt, and
sediment. Drainage basins lose water and sediment through
evaporation, deposition, and streamflow. A number of factors influence
input, output, and transport of sediment and water in a drainage basin.
Such factors include topography, soil type, bedrock type, climate, and
vegetation cover. These factors also influence the nature of the pattern of
stream channels (Figure 10aa-2).
Figure 10aa-2: Common drainage pattern types.
Trellised drainage patterns tend to develop where there is strong
structural control upon streams because of geology. In such situations,
channels align themselves parallel to structures in the bedrock with
minor tributaries coming in at right angles. Areas with tectonic faults or
bedrock joints can cause streams to take on a grid-like or rectangular
pattern. Parallel drainage patterns are often found in areas with steep
relief or where flow is over non-cohesive materials. Dendritic patterns
are typical of adjusted systems on erodable sediments and uniformly
dipping bedrock. Deranged drainage patterns are found in areas recently
disturbed by events like glacial activity or volcanic deposition. Over
time, the stream will adjust the topography of such regions by
transporting sediment to improve flow and channel pattern.