IN MARCH 1996, flood waters raged through Arizona’s Grand Canyon for the first time in over thirty years. Water rushed up beaches usually reserved for campsites, small trees drowned and rapids vanished from sight. For a week, the entire Colorado River was transformed into a turbulent monster.
The awe-inspiring flood was not a natural event. The water had been released from an upstream dam to reproduce the sort of flood the Grand Canyon would have experienced every year during winter and spring rains, before the river was dammed in 1963.
The reason for the controlled flood was not nostalgia. It was an attempt to undo years of environmental damage caused by the dam. Although it didn’t go entirely to plan, it was mostly a success and the lessons learned will be invaluable in planning the next flood, which is scheduled for later this year. The hope is that this flood will make a really good job of restoring the Grand Canyon to its pre-dam condition. That’s if short-sighted politics doesn’t prevent the flood happening at all.
Advertisement
Dams have been on the Colorado River for nearly a century. However, there were no dams upstream of the Grand Canyon until 1963, when the Glen Canyon Dam was built. At the time, the idea of damming the river above the canyon didn’t alarm anyone: it was a dam-building era and cheap, clean hydroelectricity seemed like a good thing.
But by the late 1970s, serious long-term effects were becoming apparent. Downstream of the dam, the Grand Canyon was suffering from a dearth of new sediment. People rafting down the river found the beaches they were used to camping on had eroded to half their original size. Meanwhile, the National Park Service noticed that the Colorado River’s largest fish, the pikeminnow, had completely disappeared from the river system. And the humpback chub, a fish native to the Colorado, was endangered. Alien carnivorous fish such as brown trout and rainbow trout had spread through the entire river system, as had a non-native river tree called the tamarisk.
The reason for the geological changes was easy to identify. Before the dam, river water in the canyon carried so much sediment that the river actually had a cloudy rust colour, which earned it the name Colorado – Spanish for reddish – as it flowed through an area that used to be part of Mexico. But now the water is forced to come to a halt behind the dam before being released into the Grand Canyon. Over 90 per cent of the river’s sediment is dropped here, leaving the river crystal clear. This is bad news for young humpback chub, which use cloudy water to hide from predators such as the carnivorous trout. Clear water makes the chub an easy target.
The lack of sediment also explains the beach erosion. Beaches depend on annual floods bringing a continuous supply of fresh fine sand, but the dam traps the majority of this. Small tributary rivers flowing unimpeded into the Colorado River below the dam bring some sediment, but not enough.
The dam has caused other geological problems. By limiting the river to a steady low flow all year round, the dam ended the canyon’s seasonal floods. Rapids, which depend upon flood waters to clear any debris from between the boulders, became choked.
These conditions probably set the chub back further, because they like constant, turbulent waters. Trout, on the other hand, thrive in the clear, placid pools between the rapids. “Chub’s decline over the past 10 years correlates with increases in the trout population,” says aquatic ecologist Michael Yard of the US Geological Survey’s Grand Canyon Monitoring and Research Center in Flagstaff, Arizona. Yard says the chubb suffered and the trout thrived most when demand for power was low and less water was run through the turbines into the canyon.
In 1989, under growing public concern and pressure from environmental groups, the US Bureau of Reclamation in Washington DC sent a task force into the canyon. The result was an environmental impact statement highlighting a multitude of problems in the Grand Canyon ecosystem. The team said that sediment, introduced by tributary rivers, was collecting along the Colorado’s bottom due to the restricted water flow. This triggered a stunning and, at the time, revolutionary idea. Why not try to reverse some of the decline by opening the dam for a short time so that flood waters stir up the sediment on the river bed? This should, the team claimed, help rebuild the sandbars and clean the silt out of the backwater channels used by native fish. In July of 1995 the desperate bureau gave the go-ahead.
The punching of a few buttons on 26 March 1996 opened the floodgates, allowing 0.7 trillion litres of water to bypass the turbines and, at a cost of $2 million in lost electricity revenue, flow into the canyon at a rate of 1300 cubic metres per second for a week, just under half the rate at which the river used to flood. The National Park Service issued safety warnings and monitored the canyon.
Geologists hoped the flood would recirculate fine sediment at the bottom of the river. Ecologists hoped the flood might also sweep away young trout, which aren’t adapted for flood conditions, and clear the way for a chub recovery by reconstructing their native habitat. “It was an enormous science experiment,” says Robert Webb, a hydrologist at the USGS in Flagstaff.
In the weeks after the flood waters cleared, researchers were amazed by what they saw. The flood seemed to have restored the canyon to a near pristine state. Beaches that had been rocky and barren of sand turned into beautiful sandy hills. This presumably meant that flood waters also improved the shape of the river bed. Instead of sediment sitting at the base of the river, it had been piled high into beaches and sandbars.
The most impressive changes were seen in the rapids. The debris and sediment that had been choking them off was gone. “A lot of rapids were becoming quite dangerous to the [white-water] rafters,” says Webb. “The flood cleared a lot up.” In particular the canyon’s largest rapid, Lava Falls, saw a startling transformation. Debris that had been constricting its white water for over a year was dislodged, increasing the width of the rapid by an average of 5 metres.
However, while many of the flood’s positive effects have lasted with remarkable resilience since 1996, not everything went as planned. This summer, David Rubin and David Topping, also at the USGS in Flagstaff, published work showing that the badly eroded sandbars were not rebuilt using sediment from the whole river bed (Eos, vol 83, p 273). Instead, the sand came only from the edges of the river bed, at the base of the same sandbars, making them narrower but higher. The river may have looked better from the bank but this was at the expense of the views from the river bed. “It was a bit like using your credit card to bump up your bank account,” says Topping. The same applied to beaches, which are just sandbars above the waterline.
The flood brought similarly mediocre results for the endangered species. While the chub were not harmed in any significant way by the flood, neither were the competing trout. And the raging waters actually exacerbated the tamarisk invasion, spreading its seeds all along the canyon’s banks.
So although the idea behind the flood – that it could be used to recirculate sediment – was viable, this was tempered by the discovery that not nearly as much sediment was available at the base of the river as thought. To explain this, researchers looked back at the history of flows released during the 1980s and early 90s. Although the floodgates had remained closed, power companies had released extra water through the turbines whenever demand for power was high. It now seemed likely these flows were fast enough to pick up sediment and carry it downstream, eventually out of the canyon. But their water level was too low for the sediment to reach the tops of sandbars and beaches, says Matt Kaplinski, research associate at Northern Arizona University in Flagstaff.
Other surprises were buried in the reams of data generated by the 1996 flood. Photographs and measurements of beach and sandbar size suggested that most of the rebuilding occurred at the start of the flood. After that, the flood waters actually eroded the beaches away. Flood waters are the only force that can get sediment from the river bottom and use it to build beaches, but like any fast-flowing water, they also move sediment downstream.
Experiments, however, are all about trial and error, and the canyon’s geologists are using everything they learned from the 1996 flood to guide the 2002 one. They now know that the next flood only needs to last for two days. They also know that the dam should be kept at a very low output for a period of several months before the flood, so that sediment from downstream tributaries, such as the Paria River, can build up on the river bed.
This looks like the best way to fix the canyon’s geological problems. What about the ecological issues? Running the flood in winter has a second advantage: the invading tamarisk river tree will not be in seed, so the torrent won’t spread the tree, and could even damage smaller tamarisks. However, the discussion is complicated. The tamarisk may compete with native plants, but it also provides a superb nesting habitat for an endangered bird, the Southwestern willow flycatcher. In spite of the name, the birds seem to prefer tamarisk to willow. “Is a species automatically bad if it is non-native?” asks Webb.
As for the unwanted trout, Yard is keen to follow up the winter flood with a series of high flows in the spring, when the trout are reproducing. These would go through the turbines instead of the floodgatesand flush through sediment that geologists would rather was on the river bed. These flows should hit the trout during their spawning period, hopefully enabling the chub to recover. But geologists are also worried that repeated spring floods could erode away fresh deposits. “It is possible that these spring flows could undo everything the winter flood builds,” says Kaplinski.
Agreement between scientists is elusive enough but the team that makes the decision on whether the dam should be opened includes representatives from states along the Colorado River, Native American tribes, fishers, river rafters, environmentalists and, of course, power contractors. More formally known as the Glen Canyon Dam Adaptive Management Program, the team recommends to the federal government how the dam should operate.
Not everyone in the group is backing the flood. The State of Colorado, whose rivers feed the canyon, may try to fight any decision to flood in the courts, citing concerns over water conservation. No scientists support their concerns, but it’s not clear how a US administration with the stated aim of making the country more energy independent will react to this pressure. But Randy Peterson, manager of the Adaptive Management Program, thinks Colorado will probably withdraw its opposition before the argument reaches that level. “We believe we can keep law suits out of the picture,” he says.
The good news is that the power companies that share in the dam’s operation have agreed to the flood, despite the fact that they’ll incur greater losses than in 1996. According to Peterson, the companies won’t just lose around $2 million during the controlled flood itself. The reduced flow throughout the preceding autumn will allow only a small portion of the hydroelectric generators to be used, at a cost of roughly $8 million. But if the series of high flows in the following spring get the go-ahead, the companies might be able to recoup some of their losses by running more water through than normal.
Just like the 1996 flood, the 2002 one is an experiment. But if a few months of restraint followed by a two-day torrent can undo years of ecological destruction, environmentalists may clamour for floods to become a regular fixture, and not just at the Grand Canyon. Jeff Mount, director of the Watershed Center at the University of California, Davis, is watching the events closely. “California has more than 1400 dams,” he says, every one of which has sediment issues. “We need experiments like the ones they are doing at the Grand Canyon to give us the courage to try our own.”
But what if it doesn’t work? If the tributary rivers don’t bring enough sediment into the system, researchers will have to get some elsewhere. One possibility is to dredge the sediment caught behind the dam and dump it on the other side, before attempting another deliberate flood. But tests on this sediment have indicated it is high in naturally occurring heavy metals such as selenium. Geologists may have to consider the expensive option of bringing sediment in from elsewhere.
In the end, though, it’s unlikely that huge power consumption comes without some environmental cost – whether that’s to the geology or the wildlife of the canyon. Though researchers may be able to mitigate the damage by controlling the dam’s operations, the only sure way to return the canyon to its natural state is to decommission the dam.
