Environmental Engineering Reference
In-Depth Information
10
1
10
1
A
B
10
0
10
0
10
-1
10
-1
10
-2
10
-2
1/27/90
5/7/90
8/15/90
11/23/90
3/25/60
6/11/68
8/28/76
11/14/84
10
0
10
0
C
D
10
-1
10
-1
10
-2
10
-2
10
-3
10
-3
1/27/90
5/7/90
8/15/90
11/23/90
3/29/86 8/11/87 12/23/88
5/7/90
9/19/91
10
5
E
10
4
10
3
10
2
7/4/96
8/13/96
9/22/96
11/1/96
12/11/96
Date
FIGURE 5.6
Hydrographs from three river systems plotted on log scales. The Niobrara
River in Nebraska is mostly spring fed and shows relatively little variation in discharge among
(A) and within years (B; note only about a 10-fold difference in each year, whereas two or
three orders of magnitude are covered in the remaining hydrographs). Kings Creek in Kansas
is a small, intermittent, prairie stream, with alternating periods of wet and dry over the years
(C). A typical year in Kings Creek includes both times of no flow and floods (D; note 0.001
0 discharge in C and D). A stream in a steep watershed (Slaty River on the west coast of
New Zealand) with frequent rainstorms exhibits approximately weekly floods (E) (data from
A and B courtesy of U.S. Geological Survey; data from C and D courtesy of Konza Prairie
Long-Term Ecological Research project; and data from E courtesy of Barry Biggs and Mau-
rice Duncan).
5.6C and 5.6D). Steep watersheds and intense storms can lead to great
variability in discharge (Fig. 5.6E). Damming changes the natural hydro-
graph in several ways. Generally, floods are moderated and low discharge
periods may be more rare (Fig. 5.7). If the dam is used for power genera-
tion there can be fluctuations in downstream discharge relative to changes
in power demand throughout the day, but much of the seasonal variation
in flow can be decreased.
Fluctuation in water discharge has been used to characterize streams
and linked to the community structure of organisms (Poff and Ward,
1989). In this approach, discharge variability, flooding patterns, and extent
of drying are used to create a classification system (Table 5.2). This is one
approach that can be used by stream ecologists to describe stream charac-
teristics and relate them to effects on the organisms that inhabit them.
Streams can also be characterized by their surrounding landscape and
the associated vegetation. Thus, scientists speak of desert streams, forest
streams, or arctic streams. This classification method can be useful because
the terrestrial vegetation in the landscape that the streams drain may drive
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