River processes in the Colorado river are to be artificially managed in order to help re-build
sandbanks and ecosystems devastated by the building of the Glen Canyon dam in 1963.
The Colorado is already one of the most highly managed rivers in the world. Feeding seven US states and
Mexico, the natural regime of the river was completely altered during the Twentieth Century. The Hoover
Dam (1935) and the Glen Canyon Dam (1963) store enormous amounts of water, reducing peak
discharge levels of the river and providing a source of water than can then be transported, via man-made
constructions such as the Colorado Aqueduct, to thirsty desert cities like Las Vegas. However, these
direct changes to river flow have had indirect impacts upon river processes and river valley landforms.
Now, US government agencies are introducing new management measures aimed at modifying river
processes (erosion, transport and deposition) to help restore natural sandbanks and ecosystems.
The Colorado’s regime
The regime of a river is its pattern of flow over a period of time, usually one year.
(1) The Colorado’s discharge is naturally very uneven with a marked summer maximum and lower flow
between September and April. The marked unevenness of summer and winter flow is attributable to its
massive drainage basin. The Colorado covers seven US states, including Wyoming, Utah, Colorado and
Arizona, within which a number of mountain ranges lie. It is the summer melt of snows in the Rockies, the
Wind River Mountains and the Unita range that produce large amounts of overland flow in early summer
and a pronounced rise in river levels. In extreme years of the early Twentieth Century, discharge rose from
200 to 2,600 cumecs at Lee’s Ferry, prior to the construction of the Glen Canyon dam. During some years,
a second peak in flow naturally occurs in the autumn around October, due to convectional rainstorms over
the Colorado plateau.
(2) However, water management schemes have radically altered the natural regime of the Colorado. Flood
peaks have been smoothed out by dam building. The Hoover Dam (1935) can store the equivalent of two
years’ river flow in Lake Mead. At Lee’s Ferry in Arizona, just downriver from the Glen Canyon Dam (1963),
the mean annual flood has been reduced from 2,264 cumecs to 764 cumecs.
Source: Hilton, K. (1985) Process and Pattern in Physical Geography, Unwin Hyman, London, 108-14
Why is there a need to manage river processes?
The Glen Canyon dam was built approximately half-way along the course of the Colorado in Arizona and
was completed in 1963. Its construction had the following impacts upon river processes and landforms:
o
Clearer water By functioning as a sediment trap, the natural silts and sands of the Colorado are
prevented from travelling further downriver. Rates of transportation of sediments (both suspended
load and bed load) are reduced to nearly zero. Below the dam, in Lee’s Canyon, the load of the river
has been reduced by a factor of 200. Solutes can, of course, still be transported as they have been
dissolved and are not trapped by barriers.
o
Disappearing sandbanks In turn, these transported sediments used to help build sandbanks lower
down the river. As a result, changes have occurred to the Colorado’s alluvial morphology (the
landforms that are dependent upon river deposits). Sandbanks along the sides of the river – often
forming the slip-off slopes of meanders, have been starved of sediment and have been reduced in
size.
o
Colder water The river water now has a much lower sediment load and is colder as a result, as
transported particles can absorb heat (just as land heats up more quickly than water when the sun
shines).
o
Vanishing species Shrinking sandbanks and colder water have modified the river ecosystem, leading
to the loss of four of the Colorado’s natural fish species, including the squawfish. These species only
spawn near sandy bars and thrive in silty, warm waters. They are also preyed upon by trout, who
prefer clearer, cooler water and whose numbers have risen accordingly since 1963. In addition,
plants and animals that live on the sandbanks –such as the Kanab snail and the willow flycatcherhave suffered decline.
o
Fishing and rafting The sandbanks also had human uses – for fishing and rafting – that cannot be
pursued any more in some parts of the river.
These three sets of changes - shrinking sandbanks, vanishing species and the loss of a valuable leisure
resource – mean that there is now a need to manage river processes and to attempt to restore the
river’s natural equilibrium.
How are river processes being managed?
Following the recommendation of scientists, four giant steel pipes are now allowing the release of
controlled amounts of water from the dam following periods of heavy rain or snow-melt. Run-off brings
plenty of sediment into the dam at certain times of year. However, it quickly begins to settle at the
bottom of the dam. There is already a depth of at least 20 metres’ sediment throughout much of the dam
(it is predicted to fill up entirely within four hundred years!) When new inputs of run-off arrive in the dam
- bringing fresh sediments while also disturbing existing deposits - it is vital that the water is released
quickly before sedimentation (re-settling of particles) can take place. For instance, late autumn
thunderstorms in October led to the decision to release water this week (The Independent, 24 November
2004). The following will hopefully be achieved:
o
Higher discharge Discharge will temporarily rise to 41,000 cubic feet per second. This will provide
the energy needed to transport large amounts of suspended silts, sands and gravels.
o
Increased transportation Over the 90-hour operation, 800,000 tons of sediment will be transported
out of the Glen Canyon dam (this may, in turn, lead to higher rates of erosion near to the dam as
there will be increased river energy and an abundance of suspended abrasional material - scientists
will need to monitor carefully).
o
Increased deposition Further down-river, in sections where the Colorado’s channel shape changes to
become wider and shallower, deposition will occur as the river loses energy due to its decreased
hydraulic radius.
o
Natural system state restored As the river deposits its load sequentially (heaviest first), the
sandbanks will begin to be restored. In the long-term it is hoped that the Colorado ecosystem may
be restored closer to its original state and that one currently vanishing fish species – the humpback
chub – will be saved.
Governance note: the cost of this scheme has been $3.7 million. It was supported by a partnership of a
dozen government agencies, such as the Department of the
Interior and the U.S. Geological Survey.