USGS Outstanding in the Field - Not Enough Beaches in Grand Canyon?

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Narrator: Welcome, and thanks for joining us for another episode of Outstanding in the Field, the U.S. Geological Survey’s podcast series produced by the Ecosystems Mission Area. We highlight our fun and fascinating fieldwork studying ecosystems across the country. I’m Marisa Lubeck.

Today we are talking about beaches in a place that most people probably would not think of—the Grand Canyon. USGS scientists at the Southwest Biological Science Center’s Grand Canyon Monitoring and Research Center in Flagstaff, Arizona are looking at how a dam and vegetation are making things difficult for the beaches in Grand Canyon.

Paul Grams: “People refer to sandbars in Grand Canyon as beaches commonly, and that’s because that is what they are to most visitors. At the end of a day, people on river trips pull up to a sandbar, tie their boats up to a riparian tree or shrub if convenient, they set up camp, they play frisbee, they play bocci ball, they cook dinner. Aside from the sandbars, it’s a pretty rough and rocky place down there and difficult to find a place to camp.”

Narrator: That’s Paul Grams, a USGS research hydrologist describing how sandbars provide a beach-like setting for river goers along the Colorado River in Grand Canyon. Another important feature that is quite common along the sandbars, but does not make for comfortable recreation areas in the same way that bare beaches do, are stands of vegetation along the river, or riparian vegetation. USGS geologist Joel Sankey doesn’t want that vital river resource to be taken for granted.

Joel Sankey: “People should care about riparian vegetation in the Grand Canyon because riparian vegetation provides important habitat for birds and for animals. It also provides important habitat for people, in particular, it provides some shade from the hot desert environment and sun. Riparian vegetation is a lush, green oasis of plants that rely on the Colorado River, and water on the river specifically to survive. The riparian zone, where riparian plants grow along the river in Grand Canyon, is really in stark contrast to the surrounding hot desert of the Grand Canyon and the Mojave and the Colorado Plateau Deserts that surround the canyon.

Narrator: Riparian plant communities are often biodiversity hotspots, especially in water-limited regions like the Southwest. But what happens to these areas when a 710-foot-tall dam is constructed on the Colorado River upstream of Grand Canyon?  Here’s Paul Grams.

Grams: “There are two really big ways that the dam has affected sandbars in Grand Canyon. First, Lake Powell, the reservoir that’s formed behind Glen Canyon Dam is nearly 200 miles long. All the sediment that used to flow down the Colorado River and the San Juan River from the Upper Colorado River Basin is trapped in the upstream end of Lake Powell.”

Narrator: What that means is the Colorado River downstream of the dam has been transporting only about 6-10 percent of the amount of sediment, mostly sand, than it used to before the dam. And that creates a shortage of sand for rebuilding sandbars, or the beaches of Grand Canyon.

Grams: “The other issue has to do with the flows that are released from the dam. The sandbars used to be rebuilt every year during the spring flood. Without floods, sandbars don’t get replenished and just erode, and to top that off, the low flows released from the dam are higher than from the pre-dam period, and those flows often fluctuate daily for generation of hydroelectric power. Those fluctuations can cause daily cycles of erosion at the sandbars. So, there’s less sand in the system, no spring floods to rebuild the bars, and higher rates of erosion during the rest of the year.”

Narrator: We know sandbars are habitat for riparian vegetation, so we might think that sandbar erosion has made it difficult for riparian vegetation since the dam. However, as Joel explains, the dam has also influenced the river’s flow and that has consequences for vegetation.

Sankey: “From some standpoints there’s arguably too much riparian vegetation now growing along the banks of the Colorado River in Grand Canyon owing to the way Glen Canyon Dam has been operated over the past five plus decades that it’s been in existence. Riparian vegetation has increased dramatically due to the fact that Glen Canyon Dam operations have removed extreme annual floods as well as low flows of the river. The increase in riparian vegetation is now actually crowding out sandbars and campsites, it’s decreasing the area of bare sand, also an important habitat type in Grand Canyon.”

Narrator: The elimination of very strong floods that used to occur during the spring, and sometimes during monsoon storms, has allowed riparian vegetation, especially woody plants, to colonize sandbars. Before the dam, the floods would either scour vegetation off sandbars or bury them under sand.    

Grams: “We monitor the conditions of the sandbars by going in the field and making measurements and taking photographs at a collection of study sites. We do this every year and throughout the year to keep track of how fast the sandbars are eroding and to evaluate the effects of periodic controlled floods, which we also call High-Flow Experiments. We now have these high flows just about every year, and they are working to rebuild many of the sandbars. These floods are not the same as pre-dam floods. They’re half the flow magnitude or less, much shorter in duration, and to take advantage of the periodic supply of sand from tributaries, they occur in the fall instead of the spring. So, we can’t expect the beaches to be the same as in the past, but it seems that these high flows result in some improvement over having no floods at all.”

Narrator: In addition to this intensive monitoring program, USGS scientists are investigating other ways to help the National Park Service manage the sandbars in Grand Canyon. 

Sankey: “Our research in Grand Canyon is looking at how targeted riparian vegetation mitigation treatments for the National Park Service, that intentionally removes riparian vegetation from sandbars, can be used in conjunction with controlled floods, to increase bare sand habitat at Grand Canyon. This has important implications for people because it can benefit recreationists like river rafters who camp in Grand Canyon. It’s also important for controlling invasive plants that expand onto these sandbars. The combination of these targeted riparian mitigation treatments, vegetation removals, and controlled floods, is also important because it can help the National Park Service manage important cultural resources like archeological sites that are hundreds of years old that the National Park Service is trying to preserve in their natural environment.” 

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This podcast is a product of the U.S. Geological Survey, Department of the Interior. A special thanks goes out to Paul Grams, Joel Sankey, and Todd Wojtowicz from the USGS Southwest Biological Science Center, and the rest of the Outstanding in the Field team: Suzanna Soileau, Hannah Hamilton, Sue Kemp, and Catherine Puckett. And now you can follow us in Instagram at USGS_Wild.  I’m Marisa Lubeck. Thanks for listening.

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