Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 48

Rivers are controlled or disturbed not only by direct low and channel modiication, such as was
previously noted, but also indirectly by changes in their watershed and underlying groundwater. At
issue, for example, is the natural low hydrograph. Changes in the watershed impact the timing and
magnitude of runoff. Changes in the groundwater impact recharge. Since the goal of restoration is
to return the ecosystem to its undisturbed condition, the undisturbed characteristics should extend
not only to the river's bed but also to the natural low hydrograph. Unfortunately, because of the
extent and long history of the modiication of rivers and watersheds, there is often little information
available to deine and design the undisturbed or natural low hydrograph.

3.2 FLOW MODIFICATION STRUCTURES

3.2.1 d aMS
A dam is a barrier that is constructed to contain or constrain the low of water or divide water,
such as between freshwater and saltwater. Dams vary in size from those in small farm ponds or
detention ponds in urban development projects to some of the largest engineered structures in the
world. The impact of a dam, such as that on a river, is to increase the storage of water upstream
and consequently increase the water-surface elevation. The extent to which the increased eleva-
tion extends upstream is called the backwater and typically varies with time and with the reser-
voir storage (e.g., as it impacts lake levels). The water-surface elevation within the zone is latter
horizontally (known in hydraulics as the M1 backwater curve; Figure 3.1) and in the reservoir
pool the water is deeper and the velocities lower than in the upstream river.
The release from the dam may be controlled (such as for hydropower) or not (such as over a spill-
way) or both (such as hydropower with an emergency spillway). The impact of uncontrolled storage
is to attenuate the peak lows in releases from that in the inlows. The dam impacts the physical,
chemical, and biological characteristics of the impounded waters and the downstream receiving
waters, either or both being impacted positively or negatively as will be discussed later.

3.2.1.1 So How Many Are There?
Because of the importance of dams, and the consequences associated with their failure, a National
Inventory of Dams (NIDs; Figure 3.2) computer database is maintained by the U.S. Army Corps of
Engineers, authorized by the National Dam Inspection Act (P.L. 92-367). The web-accessible database
tracks information about a dam's location, size, use, type, proximity to the nearest town, hazard clas-
siication, age, and height. There are approximately 76,000 dams over 2 m and 2.5 million smaller dams
on U.S. rivers. According to the U.S. Fish and Wildlife Service (www.rivers.gov), large dams across

M1 backwater curve: nonuniform
flow where the water slope does
not equal the bottom slope and
approaches zero

Steady uniform flow
(water slope =
bottom slope =
constant)

Datum

FIGURE 3.1 M1 backwater curve.