Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 69

Lag time before
urbanization
Lag time after
urbanization
Q after
Q before
Time (h)
FIGURE 3.29 A comparison of hydrographs before and after urbanization (blue bars indicate rainfall rate
and timing). The discharge curve is higher and steeper for urban streams than for nonurbanized streams
due to faster and greater runoff. (From FISRWG, Stream Corridor Restoration: Principles, Processes, and
Practices , Federal Interagency Stream Restoration Working Group, 1998.)
time, which from an ecological perspective may be just as important. Therefore, the natural low
hydrograph is altered.
The impact of urbanization is not only a change in the magnitude, timing, and duration
of lows, but also a change in the runoff quality. For example, temperatures as well as sedi-
ment loads may be altered. As indicated in Section 3.3.2, a change in the sediment load may
result in destabilization of the river or stream, such as changing it from a Stage I to a Stage II
system, under the channel evolution model of Simon and Hupp (1986) (Figure 3.21), with the
consequence of increasing sedimentation and erosion. Runoff of pesticides and herbicides from
lawn maintenance, petroleum products and metals that accumulate on roadways and parking
areas, and nutrients from fertilizers and other sources and materials has also increased due to
urbanization.
3.5 ABSTRACTIONS AND AUGMENTATION
In addition to instream structures or channel modiications, the low in rivers is often regulated based
on abstractions or augmentation. Abstractions, or withdrawals, result in a reduction in discharge and
also, commonly, a change in the timing or shape of the natural low hydrograph. The common uses
of water withdrawals include meeting domestic, agricultural, industrial, and other demands. As
indicated in Chapter 1, thermoelectric power (48%), irrigation (34%), public water supply (11%),
and industrial uses (5%) account for 98% of the U.S. water use (Hirsch 2006, based on data from
Huston et al. 2000). Of these water uses, some may be consumptive (abstracted water not directly
returning to the waterbody) and some not. For example, thermoelectric power is used in generating