Environmental Engineering Reference
In-Depth Information
on releases from the dam. If the dam does not release, such as during drought periods, the tailwater
cold-water isheries are diminished or cease to exist due to lack of nutrients and increases in temper-
atures. Therefore, minimum low requirements are often established for such structures protective
of the downstream isheries. Similarly, minimum lows are established to protect other downstream
uses, such as water supply.
An informative example of studies resulting in the establishment of minimum release require-
ments is that for the White River system in Arkansas. The White River system consists of ive U.S.
Army Corps of Engineers (the Corps) project dams (Bull Shoals, Norfork, Beaver, Table Rock, and
Greers Ferry lakes) built between 1941 and 1965 and authorized for lood control and hydropower,
and, to a lesser degree, water supply. Each of these reservoirs has highly productive tailwater trout
isheries and a well-developed tailwater ishing and tourism industry. Since management of the tail-
waters was not a component of the original authorized use, and implementation of minimum lows
would reduce income from hydropower, the implementation of minimum lows required years of
study and several acts of Congress to implement. After many years of efforts to establish and imple-
ment minimum tailwater lows, the Water Resource Development Acts (WRDA) of 1999 (Section
374) and 2000 (Section 304) irst modiied the basic authorization and operation for the ive multi-
purpose reservoirs “to provide minimum lows necessary to sustain tail water trout isheries, and
authorized feasibility studies” (USACOE 2009). An environmental impact study was required as
well as studies to determine the costs and mitigation measures for losses of income due to hydro-
power. Once these studies were completed, the Energy and Water Development Appropriations Act
(H.R. 3183) allocated funding in 2006 (Section 132) for Bull Shoals and Norfork lakes, but did not
authorize the implementation of the proposed minimum lows at Beaver, Table Rock, or Greers
Ferry lakes. Construction was initiated with the implementation of minimum low releases from
Norfolk and Bull Shoals Dams beginning in 2012-2013.
One problem with minimum low requirements is that they vary with the particular use of a
waterbody, and the uses are often in conlict. For example (available at Watershed Academy Web:
Protecting Instream Flows, http://www.epa.gov/watertrain):
•
Pollutant Concentration: Higher lows are important for the dilution of pollutants; in fact,
many rivers and streams that violate water quality standards for common pollutants do so
when lows are abnormally low.
•
Aquatic Habitat: Pools, runs, and secondary channels are deeper, more varied, and more
abundant when the low is higher, and this allows a river or a stream to support more abun-
dant and diverse aquatic life.
•
Water Temperature: The amount of water in a stream or a river affects its resistance to
becoming warm, because more water takes longer to heat. Higher lows protect sensi-
tive, cold-water species, such as trout and salmon, from harmful or even lethal water
temperatures.
•
Recreational Uses: Sports such as white-water rafting and canoeing depend on certain
levels of low for the number of days per year that outitters can make a living. Flows also
signiicantly affect other sports such as ishing.
While minimum lows are still used in water management, the current environmental low para-
digm asserts that all lows are important (King et al. 2003; Hirsch 2006) and that healthy aquatic
and wetland populations and communities require variable low regimes to protect habitats and life
history processes (Poff et al. 1997; Richter et al. 1996, 1997; Neubauer et al. 2008; see Chapter 4).
The biota of river ecosystems have evolved in response to natural variations (Bunn and Arthington
2002) and a variety of management strategies have been developed to evaluate “how much water
a river needs” and to protect the low regime. One adaptive management method described by the
U.S. EPA is the range of variability approach (RVA; available at: http://www.epa.gov/watertrain) as
discussed in the following section.
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