|
f
<br />Water Quality
<br />Because the water - intake structures of Glen
<br />Canyon Dam are well below the surface of
<br />Lake Powell, where the warmth of the sun
<br />cannot penetrate, water released from the
<br />dam is cold, with an average temperature of
<br />about 48 °F (9 °C) for the period from 1988
<br />to 2005. Warming occurs as the water moves
<br />downstream, reaching an average annual
<br />high of about 64 °F (18 °C) at Diamond Creek
<br />between 1990 and 2002. Low temperatures
<br />have threatened the survival of endangered
<br />humpback chub (Gila cypha). Beginning in
<br />2003, an ongoing drought in the upper Colo-
<br />rado River Basin has reduced the water level
<br />in Lake Powell, meaning releases are drawn
<br />from closer to the surface of the reservoir
<br />where the water is warmer. As a result, water
<br />temperatures have increased substantially,
<br />reaching an annual high of 70 °F (21 °C) in
<br />2005 at Diamond Creek. The warmer releases
<br />may be playing a role in recent increases
<br />in native fish, including humpback chub in
<br />Grand Canyon (see below).
<br />Specific - conductance data are collected as
<br />a cost - effective method for estimating salin-
<br />ity, or salt inputs, which affect ecosystem
<br />functioning as well as municipal, industrial,
<br />and agricultural water users. Drought condi-
<br />tions, prevalent since 1999, generally result
<br />in an increase in specific conductance in
<br />Lake Powell and water released downstream,
<br />owing in part to the decrease in the volume
<br />of water in the reservoir.
<br />Sediment
<br />Throughout Grand Canyon National Park,
<br />sandbars create habitat for native plants and
<br />animals, provide camping beaches for river
<br />runners and hikers, and act as the source of
<br />sediment needed to protect archeological
<br />resources. Sandbars in the park have been
<br />significantly eroded because Glen Canyon
<br />Dam traps all of the upstream sediment sup-
<br />ply and eliminates natural flooding.
<br />Today, Colorado River tributaries below
<br />the dam are the only sources of sediment
<br />available to maintain the park's sandbars,
<br />providing just 16% of the sand supply avail-
<br />able before the dam's construction. In 1996,
<br />2004, and 2008, short- duration high -flow
<br />experimental water releases, which simulate
<br />mild natural floods, were tested for their
<br />ability to maintain sandbars with this post -
<br />dam sand supply. High -flow releases are re-
<br />quired to transfer sand from the riverbed and
<br />low - elevation parts of sandbars to higher-
<br />elevation environments that are only inun-
<br />dated during higher than normal releases.
<br />The 1996 high -flow experiment re-
<br />sulted in limited areas of sandbar building
<br />at the expense of larger areas of erosion,
<br />whereas the 2004 release produced net
<br />sandbar building in the upper reaches of the
<br />277 -mile -long river system. As a result of
<br />the 2004 high -flow experiment, scientists
<br />confirmed that increases in total sandbar
<br />area and volume can occur when high -flow
<br />releases follow tributary floods that enrich
<br />sand supplies in the main channel below
<br />the dam. In 2008, when sand supplies in the
<br />main channel of the Colorado River were
<br />approximately three times larger than in
<br />2004, a high -flow experiment was done to
<br />determine whether the conditions of greater
<br />sand enrichment would result in more sand-
<br />bar building than occurred in 2004.
<br />Building on what has been learned
<br />through experimentation, scientists have
<br />concluded sandbars can be potentially rebuilt
<br />using short- duration high flows following
<br />each average to above - average input of sand
<br />from tributaries; this approach would move
<br />sand from the riverbed to sandbars before it
<br />can be carried downstream. The effective-
<br />ness of this strategy rests on minimizing
<br />sand export and sandbar erosion during
<br />periods between high flows. Export and ero-
<br />sion rates are strongly dependent on water
<br />release volume and daily release patterns.
<br />For a given annual volume of water to be re-
<br />leased from Glen Canyon Dam, the optimal
<br />dam operation for accumulating tributary-
<br />supplied sand is a constant, steady flow over
<br />the entire year.
<br />Aquatic Food Web
<br />Aquatic food resources play an important
<br />role in the distribution, population density,
<br />and growth of native and nonnative fish.
<br />Found in Lakes Mead and Havasu in January
<br />2007, the nonnative quagga mussel (Dreis-
<br />sena bugensis) will in all likelihood make
<br />its way to Lake Powell. Introductions of
<br />invasive quagga and zebra mussels (Dreis-
<br />sena polymorpha) into lakes and rivers in
<br />the Eastern United States have caused broad
<br />ecosystem -scale changes, including shifts in
<br />the location of aquatic food resources.
<br />A risk assessment by the USGS found that
<br />it is likely that the mussel will establish in
<br />moderate densities at Lees Ferry . A moderate
<br />presence of quagga mussels in Lees Ferry
<br />might actually increase food availability for
<br />fish by stimulating algae and invertebrate
<br />production. High suspended - sediment
<br />concentrations and other conditions make it
<br />unlikely that quagga mussels will become
<br />well established in the mainstem Colorado
<br />River below Lees Ferry or its tributaries.
<br />Native Fish
<br />The humpback chub is an endangered
<br />freshwater fish found only in the Colorado
<br />River Basin. Humpback chub have been af-
<br />fected not only by changes in hydrology since
<br />the construction of the dam but also by preda-
<br />tion by and competition with nonnative fish.
<br />The USGS has developed and used a
<br />computer model to assess the Grand Canyon
<br />humpback chub population status and trends
<br />from 1989 to 2008. Reproductively mature
<br />humpback chub, those 4 years old and older,
<br />appear to have decreased from 1989 to about
<br />2001, when the population stabilized at about
<br />5,000 adults. From 2001 to 2008, the adult
<br />population increased by approximately 5001o.
<br />When possible model error is considered, the
<br />estimated number of adult chub in the Grand
<br />Canyon population is between 6,000 and
<br />10,000. The most likely number is estimated
<br />at 7,650 individuals.
<br />The factors driving the estimated increase in
<br />adult humpback chub numbers in Grand Can-
<br />yon are not easy to determine. Between 2000
<br />and 2008, both human - caused and natural
<br />events have occurred that could be indepen-
<br />dently or in combination contributing to the
<br />increase. Scientists hypothesize that humpback
<br />chub may have benefited from experimental
<br />water releases from Glen Canyon Dam, re-
<br />moval of nonnative fish, and drought- induced
<br />warming of dam releases since 2002, particu-
<br />larly during late summer and fall of 2005.
<br />Grand Canyon populations of the flan -
<br />nelmouth sucker (Catostomus latipinnis) and
<br />bluehead sucker (Catostomus discobolus) are
<br />stable, and both species may have increased
<br />in the reach of the Colorado River upstream
<br />and downstream from the mouth of the Little
<br />Colorado River. In this reach, scientists have
<br />found juvenile, young, and adult fish of both
<br />species, which suggests that more successful
<br />reproduction is occurring.
<br />Nonnative Fish
<br />The number of nonnative rainbow trout
<br />(Oncorhynchus mykiss) found in Lees Ferry,
<br />which is immediately downstream from
<br />the Glen Canyon Dam and supports a rec-
<br />reational fishing industry, began declining
<br />about 2002. Research to better understand
<br />
|