ASU Research E-Magazine: What To Do About Those Dammed Rivers?

Look at the Salt River where it slices through Tempe, just north of the Arizona State University campus. it is a dried-up carcass of a waterway by this point of its meandering route, inhabited by gravel and sand. Only blocks away, researchers at ASU’s Center for Environmental Studies are speading a simple message: It doesn’t have to be this way!

Slowly, relentlessly, the Colorado River worked for the past three and a half million years. During that time the river carved a 280-mile long, mile-deep chasm into the 2-billion-year-old rock of northern Arizona. We know the river’s handiwork as the Grand Canyon. Distances across this grandest of gorges range from four to more than 15 miles.

During the past three and a half decades, operators of the Glen Canyon Dam have managed the Colorado River’s flow in order to provide hydroelectric power to humans living in Arizona and Utah. Since 1963, the dam has subdued the river’s entrance into the Grand Canyon. In addition, abrupt releases of water from massive Lake Powell, ponded behind the dam, have eroded beaches, destroyed vegetation, and killed fish downriver throughout the Grand Canyon.

Consider that the Glen Canyon Dam is only one of 431 dams built on rivers and streams throughout Arizona. Water is a precious commodity in the arid Southwest. As a result, people stockpile it for crop irrigation, electric power, and even recreation. There also is demand for water downstream from the dams. Plants and animals depend on the rivers for survival. If they don’t get enough water, they will die. It does not have to be this way.

Mimicking Mother Nature
“There’s more than enough water to go around,” claims Juliet Stromberg, an associate research professor at ASU’s Center for Environmental Studies, and an affiliated member of the botany department. “Let’s just allocate it in such a way that we don’t destroy natural areas that we appreciate.”

Stromberg studies the ecosystems of rivers. Riverside habitats are known as “riparian” ecosystems. The ASU scientist studies how the flow patterns of a river affect the plant life that grows on its shores. With such information in hand, she can provide valuable suggestions to water management professionals who are working to restore riparian habitats throughout Arizona.

“There’s been a lot of emphasis on restoration in general lately. Lots of people want to do something to protect declining habitats,” she explains. “For a few years there was a big push to plant trees. That was called restoration.”

Then came the realization that planting trees was not enough. Underlying questions needed answers. Where are all of the trees? Where have they gone? Why are the trees not establishing themselves along streams as they once did?

Arizona’s Hassayampa River flows through central Arizona near Wickenburg, about 50 miles northwest of downtown Phoenix. Cottonwoods, willows, and mesquite trees thrive along its banks. It is here that Stromberg is finding some answers.

Stromberg observes Mother Nature at work along the Hassayampa. Part of a Nature Conservancy preserve, the river is one of the few protected waterways in the Southwest today.

By studying the river’s flow patterns, Stromberg and her colleagues hope to gain a better understanding of exactly what riparian systems need in order to thrive. In turn, such information will help water management officials to better use dams to imitate natural water flows.

“We can’t go back to normal flows anymore [on dammed rivers],” Stromberg explains. “But we can mimic some of the elements of the natural flow that are important for the riparian plants and other organisms.”

Controlled Floods
Earlier this spring, the U.S. Bureau of Reclamation tried to do just that on the Colorado River. A week-long “controlled flood” released from the Glen Canyon Dam gushed into the Grand Canyon at a rate of 45,000 cubic feet per second. Duncan Patten, an ASU professor emeritus and founding CES director, served as senior scientist on the project.

“Unfortunately, we didn’t get as much of a flood as we’d have liked,” Patten says, noting a natural flood in 1983 that released 92,000 cubic feet per second. However, the controlled flood did succeed in some of its goals, the primary goal being the deposition of new sediment along the shore.

Along undammed rivers, sediment is deposited regularly with the spring floods. Along the Colorado, however, the small, sudden releases used to generate hydroelectric power actually eroded sand from the banks into the river channel. The radical water level fluctuations also exposed and dried out plants living on the river bottom. This in turn depleted an important food source for the fish in the river.

The experimental flood redeposited sediment on the riverbanks. It also created backwaters, channels of water that tend to stay warmer than the main river waters. Backwaters also serve as excellent habitats for young fish. Even the river rafting industry benefited by the rise of beaches suitable for camping.

Further south, another river may soon get a dousing as well. Pat Shafroth is a doctoral student in botany at ASU and a National Biological Service employee. He studies the ecology of the Bill Williams River, which is obstructed by the Alamo Dam.

A large portion of the land below the dam is managed by the Bureau of Land Management and the Fish and Wildlife Service, both of which strive to promote healthy wildlife habitats. Two years ago, a technical committee drafted baseline recommendations for changing water release patterns from the Alamo Dam in order to promote better fish and wildlife habitats downstream.

“What we’re trying to do is refine those recommendations and fill in some of the gaps,” Shafroth says.

Putting Together the Pieces
Shafroth’s work is much like building a puzzle with hundreds of interlocking pieces. A variety of factors contribute to an ecosystem’s health and welfare. In desert riparian areas, one of the most important pieces is flooding, which occurs periodically in Nature. In order to mimic a flood by releasing dammed-up water, several variables must be taken into account. These include:

Timing of floods: Some species, like the cottonwood and willow, require flooding to create the right conditions for their seeds to germinate. Therefore, the floods must be timed to occur before seed dispersal to provide the optimum environment for the young trees.
Magnitude of floods: The amount of water that is released also changes the growing conditions of a riparian area.
Rate of decline after a flood: When water is released from a dam to produce hydroelectric power, it often stops as suddenly as it starts. The more gradual decline found in natural systems ensures that the tiny roots of seedlings do not dry out.
Base flow: Plants need more than just a periodic flood to keep them going. The base flow maintains the underground water table at appropriate depths for plant growth and survival.

So how do researchers study the effects of these variables? After all, they cannot ask a tree how it is feeling today. Instead, they piece together clues that tell them how flood patterns affect vegetation. For example, scientists take samples of tree cores using a tool called an increment bore.

Researchers drill into a tree and pull out small cross-sections of the trunk without harming the tree. Using the sections, they can then study the plant’s history by looking at its rings. Tree ring data often is used to determine a tree’s age, but it also reveals much more of the tree’s life story.

For example, differences in the widths of the rings can show relative growth for different years. This information can be related back to records of stream flow characteristics for those years. As a result of this comparison, scientists can determine exactly how flow patterns affect growth.

Some trees are scarred during periods of extreme flooding. Scars appear as indentations in rings on the upstream side of the tree. They occur when debris slams into the tree as it surges downstream during a flood.

Balancing Act
Shafroth uses these clues and others to determine the best release patterns for a healthy riparian system. Once he gathers enough information, he will make recommendations to the U.S. Army Corps of Engineers, which manages the Alamo Dam.

Alamo Dam provides a perfect work site for Shafroth. Because the dam is used primarily for flood control, the water is not allocated for other purposes such as irrigation. As a result, managers may be able to release water from the dam in the quantity and frequency that most benefit the downstream ecosystem. This is not the case in other areas.

“Most dams are operated for purposes which limit the flexibility of their operation,” Shafroth explains. “There may be dams operated to generate hydroelectric power, or dams which impound water that is diverted for agriculture. That water is all paid for and people have rights to it.”

According to Patten, trying to appease everyone involved is “a balancing act.” At the Glen Canyon Dam, the electric generators can handle 32,000 cubic feet of water per second, meaning that any additional water released will have to bypass the generators.

“Anything that doesn’t go through the generators is considered wasted water by the electric company,” Patten says. This makes it difficult to convince dam management that they should release enough water to simulate a natural flood.

Shafroth believes that dam managers are more likely to make changes that have a minimal impact on their operations.

“There may be opportunity for some of the important [flooding factors] to be incorporated into dam operations. It would be on a case-by-case basis, and it might have to be opportunistic,” he adds.

Why so much effort to restore a river? The answers are many. The benefits extend to people as well as wildlife. One obvious reason is that rivers provide pleasant places for people to relax and enjoy themselves.

“Many of the people fighting to protect the San Pedro River [in southern Arizona] are from the big cities,” Stromberg says. “They want to have a nice place to go. We’ve already ruined the Salt River in Phoenix and the Santa Cruz River in Tucson.”

But human effects on rivers go beyond aesthetics. For example, agriculture, cattle grazing, and damming have led to a rise in the population of salt cedar, a non-native plant imported from Eurasia. Salt cedar makes the soil more saline, which impedes the growth of many native plants. In addition, salt cedar is highly flammable, making it hazardous to have around during seasonal Arizona wildfires.

Many dollars have been spent in search of means to eliminate salt cedar. One fact is known for certain: salt cedar grows better along dammed rivers than on free-flowing waterways.

Salt cedar is a new piece of the puzzle. It does not fit well into our riparian picture. As Stromberg and other scientists learn more about what should exist in a natural system, they may find even more benefits in preserving existing systems and repairing those that have been altered.

“A riparian area is there because the river functioned naturally [at one time],” Patten adds. “Simulating natural processes helps us try to maintain what was there.” —Diane Boudreau