Scientists deepen understanding of ancient rivers

Nebraska’s rivers can be seen as places for peaceful contemplation, familiar local landmarks, or valuable natural resources worth protecting.

For Jesse Korus, the state’s rivers are all of those things, but they’re also something more: They’re messengers from Nebraska’s deep geologic past. And their messages, discerned through innovative analysis of residual river channel deposits preserved in modern topography and rock outcrops, are important in helping Nebraskans understand the ongoing geologic dynamics affecting the state.

Korus, an associate professor in the School of Natural Resources at the University of Nebraska-Lincoln, is expanding knowledge of Nebraska’s geologic past by co-authoring two recent studies of the ancient river systems of the Great Plains.

“This is the first time we have directly observed the course of ancient river channels over a large area of ​​the Great Plains and associated them with past landscapes and events,” Korus said.

An article, published in Geosphere, observes that the landscapes of the Great Plains “are the product of a long-lived continental sediment delivery system” via rivers, “and yet, surprisingly little is known about these ancient rivers.” Korus and Matt’s article

Joeckel, senior associate director of the School of Natural Resources and a Nebraska state geologist, is helping to fill the knowledge gap about the history of the ancestral Platte River system, which dates back to 33 million years ago ‘years.

“Rivers were building agents of the Great Plains,” said Korus, who is also a groundwater geologist for the Nebraska Division of Conservation and Survey. Joeckel is the director of the CSD.

A second article by the two scientists, published in The sedimentary recordexplains how patterns of river sediment deposition from the Rocky Mountains to the Gulf of Mexico convey key geologic information about upstream conditions.

Huge fan-shaped sedimentary deposits in the ancient Nebraska High Plains stabilized the river’s sediment delivery system, retaining sediment for millions of years, and prevented climate fluctuations from rapidly altering the landscape in downstream areas, Korus and Joeckel wrote.

“These rivers carried not only the sediment and the water itself, but also information about the geological past, the past environment, past events,” Korus said. “For example, the uplift of the Rocky Mountains would have created a pulse of sediment that ultimately resulted in a thick body of sediment deposited in the sink” far downstream in the Gulf of Mexico.

Giant sedimentation “megafans” in ancient southwestern Nebraska, as well as parts of Colorado and Wyoming, delayed the transmission of this information by acting as a “signal buffer,” Korus and Joeckel found .

The vast fan-shaped structures, measuring up to 56 miles wide, stored sediments eroded from the Rocky Mountains for perhaps as long as 5 million years before some of it was transported downstream. This geological process delayed fluvial transmission of signals of uplift and climate change affecting the ancient river basin.

The Nebraska scientists’ analysis strengthens understanding of how rivers respond to tectonic movements, geomorphic upheavals and climate change over geologic time scales. Their work also sets the stage for Korus and Joeckel to continue a broader geologic analysis of western Nebraska, eastern Wyoming, and northeastern Colorado. The sediments exposed at the surface in these areas are analogous to the Ogallala Aquifer in the subsurface further east. This future research could improve understanding of the aquifer, Nebraska’s primary natural resource.

Such a study, Korus said, can “improve our understanding of the subsurface deposits, the very ones that are part of the Ogallala High Plains Aquifer. It therefore lends itself to a better understanding of our major aquifer systems here in the Nebraska”.

For both projects, Korus and Joeckel used remote sensing technology known as lidar, or light detection and ranging, which allows terrain analysis in remarkably fine detail. For the first paper, Korus and Joeckel identified more than 3,100 river ridges, the eroded remains of ancient river channels that provide important information about hydrology and ancient drainage patterns.

Meticulous analysis of these “fossil rivers,” spanning from 2 million years ago to about 33 million years ago, revealed the evolution of the Platte river system. It indicated that the streams of the ancient South Platte river system were initially small and diverse, following a course very different from that of the modern river system.

“The early South Platte was a lot different than it is today,” Korus said.

“Some rivers flowed northwest to southeast, perpendicular to the modern South Platte River in eastern Colorado. This suggests that this river system was not yet fully established. It was still in its infancy. early stages. Then, younger ridges show us that the rivers became larger, leading to the incision of the modern South Platte Valley. It was only after the establishment of the modern South Platte Highway that the modern North Platte Valley began to develop.

Understanding these ancient geologic details depends on discerning the messages contained in Nebraska’s geology.

“Sediments and rocks in general can be read like a book and they tell us about the geological past,” Korus said.