River management: Damming the Colorado
River management is not a new concept, but concepts in river management do undergo change. The Boulder (Hoover) Dam on the Colorado, in the south-west of the USA, was a milestone in river management. Commissioned in 1935, it marked the start of an era of big dam (over 15m high) building, not only in the USA but also globally. During the 20th century over 45,000 dams were commissioned. The peak of dam building was in the 1970s when three dams per day were being commissioned, but dam size continued to increase. The decision to build the largest dam, the Three Gorges Dam on the Yangtze, was taken in 1992.
Dams are used to provide water for agricultural, domestic and industrial purposes, for hydro-electric power (HEP), and can also control floods and improve navigation. But, as early as the 1950s, concerns were expressed over the adverse impacts, both social and environmental, of big dams. By the 1990s, the big dams debate had become a highly controversial issue and in 1998 the World Commission on Dams was established to review the performance of big dams and to assess alternative ways of developing water resources and energy. The Commission's report, published in November 2000, accepted that big dams had aided development but stated that more careful assessment was necessary before such projects were undertaken in the future. For the latest report, written five years on from 2000, please see http://www.irn.org/wcd/
Managing the Colorado
Why dam the river?
The key driver was the demand for water to develop the arid south-west of the US, a region with rainfall totals under 750mm. By 1895, southern California was short of water, not only for agriculture, but also for domestic use by the growing population. By the 1930s, power as well as water was needed to support California’s growing economy. The need for flood control had already been raised in 1905 when the Colorado demonstrated its force by drowning the Imperial Valley. Damming the river would address all three issues.
The Colorado is the most managed river in the world: over 40 dams have been built in the Colorado basin as competition for water between California and the other six basin states has intensified. Dams ensure their entitlement to water as set out in the Colorado Compact of 1922. Below Lee Ferry, six major dams – of which the Hoover Dam is the key facility – had been commissioned before Glen Canyon Dam, the key dam in the upper basin, was commissioned in 1963.
Glen Canyon Dam: A controversial issue
Long before 1956, when the decision to build the Glen Canyon Dam was taken, the dam had become a controversial issue. The creation of Lake Powell would mean drowning much of the canyon and the loss of spectacular desert landscape above the dam: ridges, mesas and buttes would become islands, and the lower ends of tributary canyons would also disappear under the water. The new town, Page, would spread across part of the Colorado Plateau close to the dam and, together with new highways, would scar the desert landscape.
Although the water storage, HEP and recreational use (3 million visitors in 1990) provided by the dam have brought wealth to the area, the lower basin states are heavily dependent on the dam to sustain their booming populations and economic growth. Not only does it provide power but also, importantly, it ensures their share of the Colorado’s water. California’s allocation – over one quarter of the river’s water – is destined for southern California, providing 60% of the water needed for Los Angeles and San Diego, and for irrigation in the Imperial Valley. But the environmental cost is also being borne by the canyons between the dam and Lake Mead. One of these is the Grand Canyon, America’s most prized natural feature.
Impact of the Glen Canyon Dam on the ecosystems of the Grand Canyon
By 1964 – only a year after the dam was commissioned – changes to the entire ecosystem in the canyons below the dam were becoming evident as a once-dynamic river was replaced by a controlled one. By 1982, alarm bells sounded. Stakeholders (i.e. those with vested interests) in the Grand Canyon were drawn into discussions as to how best to address the following issues.
- The physical characteristics of the river channel have changed. Before the dam was built, the speed and level of the river flow changed seasonally, affecting not only river processes but the duration and height to which the river’s influence was felt.
Each spring, melt water from the Rockies turned the river into a raging torrent, scouring the river channel. As the flood subsided, the high sediment load from the easily eroded soft sandstone was deposited further downstream, forming bars and wide beaches. The loss of these two important functions has been serious, being partly responsible for the decrease in the size and number of bars and beaches. Floods also cleared debris brought down by tributaries in flood, but now accumulated deposits constrict the river channel and form dangerous rapids.
This seasonal regime has been replaced by a more complex pattern of flow. The regulated discharge maintains lower levels of water throughout the year. Because the HEP plant supplies peak loads, flows fluctuate widely during the day and from day to day according to demand for electricity. But, more significantly, water levels can fluctuate by almost 4m. This, environmentalists argue, is partly responsible for the erosion of bars and beaches. - The dam has trapped silt and nutrients. Some 90% of the silt transported by the upper Colorado and its tributaries is now trapped behind the dam, reducing the supply of silt for bar and beach replenishment downstream. The clear water also erodes these features.
- The temperature of the water is now controlled. Prior to
the dam being built, water temperatures varied with the
seasons: icy cold in winter but as high as 27°C in summer.
Base water released from the reservoir is a constant cool 9°C. - These changes have impacted on vegetation and wildlife of the Grand Canyon. Cold water and loss of sand bars and beaches, which provided spawning grounds for the native hump-backed chub, have been partially responsible for a dramatic reduction in its numbers. But a major factor has been the introduction of predatory trout due to a new riverine food chain that has evolved as a result of nutrients collecting in the reservoir and clear water below the dam.
Floods ripped out vegetation at low levels on the riverside, allowing only short-lived vegetation to survive. The regulated flow of the river has allowed native willow and exotic tamarisk to gain a hold in this lower riparian zone. A complex web linking riverine and riparian zones has been created. Tamarisks have attracted insects and birds, the latter in turn attracting falcons.
How should the river be managed?
The extreme view is to decommission the dam, but this in itself would create environmental problems. Some people are content to accept the status quo: loss of some native species and gains of exotic species. But a third approach is adaptive management: maintaining a maximum flow of 25,000cfs (cubic feet /sec) and occasional floods to rebuild beaches, together with a long-term monitoring programme.
In March 1996, a controlled flood raced through the Grand Canyon in the hope that it would have the same effects as a natural flood. Initially it appeared to be a success: higher bars and beaches had been formed. But within a year these had been eroded. The flow rate – half that of natural floods – had not been strong enough to scour the river channel and produce new silt, and the remaining un-dammed tributaries, the Paria and Little Colorado, had not provided enough sediment. Instead, the bars and beaches were increased in height by the re-depositing of silt removed from their bases. There was another controlled flood in November 2004, and researchers are studying its effects: http://walrus.wr.usgs.gov/grandcan/twostudies.html (last updated January 2005)
How much longer will there be enough water?
Input
Average annual flow since 1930 14.0maf*
Current output
Loss by evaporation from reservoirs 2.0maf
Upper basin states 3.8maf
Lower basin states: California,
Arizona, Nevada and Mexico 8.2maf
Current use
Irrigation in lower basin (including Mexico)
uses 85% of allocation
Amount used by California 5.2maf
Imperial Valley alone uses (for irrigation) 3.1maf
Los Angeles and San Diego use 0.8maf
Future
Upper basin states do not fully use allocation
(7.5maf) but by 2020 developing economies
and population growth will push up use to 4.5maf
California been ordered to reduce
consumption back to allocation of 4.4maf
By 2015, with growth of Las Vegas,
Nevada to exceed allocation of 0.3maf
States not using full share will allow water-short states access to surplus
More frequent drought in the south-west will put pressure on supply
*maf: million acr
- Mesa:
- flat-topped hill with vertical or very steep sides; characteristic feature of dissected desert plateaus formed of horizontally bedded rocks capped by resistant beds
- Butte:
- similar to mesa but smaller (remnant of a mesa)
- Bar:
- deposit of sand at river’s edge, below the water level
- Beach:
- sand deposited on top of a bar, thus above water level
- Riparian zone:
- land along the river bank reached by the river
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