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Declining water levels in Lake Powell since 2000 provide a unique opportunity to study the sediments along previous shorelines. Researchers from the University of Utah, U.S. Geological Survey, and Utah State University recorded the changes in deposits, which informs understanding of hydrologic systems and the effects of reservoirs. This research, along with others, will help to build an archive of sediment records that illustrate the impacts of humans as geologic agents.

Grab a life jacket

No longer are researchers able to use a motorboat to access this northern area of Lake Powell in Utah. Whitewater rafts provide the transportation along the Colorado River to reach the sandy outcrops and previous reservoir shoreline. Running these rapids is no easy feat and require the help of professional river runners.

Once the crew and gear arrived at the beaches, they explored the newly exposed rocky side canyons and stream channel beds. These outcrops and piles of sediment have a story to tell.

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What do the sediments say?

Recording and analyzing the vertical stack of sediments is the study of stratigraphy. Each layer of sand and rock is like a page in a book. In most cases, the oldest stories are located at the bottom and the newer stories pile up on top.

Water levels in Lake Powell have repeatedly risen and fallen since the completion of Glen Canyon Dam in 1963.  When the research sites were underwater, mud and other sediments were deposited by the lake. During times when the lake level was lower than the sites, the land was exposed to the air and river channels brought in new sediments on their way downstream to reach the lake. As the researchers recorded the stratigraphy at several sites in the upper end of the Lake Powell area, they read and analyzed the story of those cycles.

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“Lake Powell acts as a tollbooth for rivers in the Upper Colorado River Basin,” said USGS Hydrologist Casey Root.  “As water from the rivers passes through the reservoir into the lower basin, sediment is left behind. In a collaborative effort between government, university, and citizen investigators, we have begun to learn how these emergent reservoir landscapes record the history of hydrologic conditions and fluctuations in reservoir level.”

Although the main takeaway was that the stratigraphy matches very well with the reservoir level history and river discharge record, this work also exemplifies the need for further investigation as drought in the southwest persists.  

The current drought represents the driest 20-year span in the last 1,200 years. As the level of Lake Powell falls in response to the drought, more reservoir sediment continues to emerge. If the drought represents the “new normal”, this study can expand greatly. New deposits left behind by Lake Powell inform our understanding of how reservoirs fill and move sediment once the lake recedes.

“Lessons that we learned at Calf Canyon can be applied elsewhere in Lake Powell as more sediment is exposed,” said Root. “It’s a unique opportunity to test fundamental stratigraphic principles in these engineered settings.”

Humans as geologic agents

In the scale of geologic time and space, it is not obvious that humans play a role impacting hydrologic and geologic systems. Reservoirs are one example of that. Recording the stratigraphic story, now available since the recession of Lake Powell in 2000, adds to the growing archive of humans as geologic agents. Research at USGS considers the complex interaction of systems, including the impacts of humans. This provides scientific information for understanding the past and useful knowledge for the future.

Learn More

Read the published article in Sedimentary Record:  https://doi.org/10.2110/sedred.2022.1.3

Press release issued by the University of Utah: https://attheu.utah.edu/uncategorized/exposed-sediments-reveal-decades-of-lake-powell-history/