Geoscience Reference
In-Depth Information
KEY CONCEPTS TO REMEMBER ABOUT
RUNOFF AND DRAINAGE BASINS
Channel
Valley wall
Streams consist of channels in which water flows down-
hill. Runoff refers to water that runs off land surfaces
into streams.
Velocity,
v
Streams are fed by runoff or groundwater, or may have
a large lake as their source.
W
i
d
t
h
,
w
Depth,
d
A drainage basin is the area that contributes runoff or
groundwater to any given stream. Drainage basins are
separated from one another by divides, which are top-
ographic rims consisting of relatively high ridges and
hills.
Drainage density refers to the relative density of streams
in comparable areas and is influenced by climate, un-
derlying bedrock, or both.
Top View
Streams are classified by their order, which ranges from
low (such as 1 or 2) to high (such as 5 or 6). Stream
order changes only at the confluence of two streams of
the same order.
Depth,
d
Slope,
s
Velocity,
v
Hydraulic Geometry
and Channel Flow
Side View
Figure 16.10 Hydraulic variables.
Streams have a measur-
able width, depth, velocity, slope (or gradient) down which
they flow and discharge. Velocity is lowest on the sides and
bottom of the channel due to the effects of friction.
Now that the organization of stream networks has been dis-
cussed, let's look more closely at the character of flowing water
within channels. This assessment falls within the framework of
stream hydrology
and focuses on the geometrical attributes (or
hydraulic geometry
) of river channels, including the measur-
able width, depth, velocity, slope, and discharge (Figure 16.10).
These concepts are respectively defined as:
Fluctuations in Stream Discharge
As far as human interactions are concerned, the most important
hydraulic variable is stream discharge
Q
, which refers to the
amount of water flowing in the channel. This value is calculated
by the simple equation
t
w
â
channel width (how wide the channel is in which the
stream is flowing).
t
d
â
depth (how deep the channel is from the water surface
to the bed).
t
v
â
velocity (how fast the water is moving in the channel;
velocity is greatest in the middle of a channel where water
flows freely and is slower on the channel bottom and sides
due to frictional forces).
t
s
â
slope (how steep the slope [also called
gradient
] is on
which the stream is flowing).
t
Q
â
discharge (how much water is flowing in the channel).
Q
â
w
ñ
d
ñ
v
Discharge is measured with a device called a
stream gauge
that can either be portable or fixed in one place in a gauging
station. The United States Geological Survey has a network of
stream-gauging stations to monitor discharge in all the major
streams in the country (see Figure 1.4 in Chapter 1). Stream
discharge is typically expressed in cubic meters per second
(m
3
/sec) or cubic feet per second (ft
3
/sec). With increases in
width, depth, and velocity, stream discharge becomes greater.
In most streams, discharge varies over the course of the
year, depending on cycles of rainfall and relative drought.
The amount of discharge that serves as the baseline level for
any stream at any given place and time is
base flow
, which
As you might imagine, these variables are intimately
related to one another in a holistic way. For example, the
velocity of a stream closely correlates to the channel slope,
with streams on steep gradients moving more quickly than
those on shallower slopes. Increased discharge naturally
leads to greater channel width and depth to accommodate
