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Study identifies sediment source in muddy Portneuf River tributary

Conservation programs may be on the horizon to help landowners along Marsh Creek in southeast Idaho address water-quality problems in the Portneuf River tributary, based on data from an Idaho State University study.
John O’Connell

Capital Press

Published on November 6, 2017 12:26PM

Idaho State University geosciences associate professor Ben Crosby stands on a cliff overlooking the confluence of Marsh Creek and the Portneuf River, near Inkom, Idaho. Crosby is part of a study that concluded bank erosion in the lower reach of Marsh Creek is responsible for heavy sediment loads, and establishment of wetlands in collaboration with willing landowners would be an effective way to improve water quality.

John O’Connell/Capital Press

Idaho State University geosciences associate professor Ben Crosby stands on a cliff overlooking the confluence of Marsh Creek and the Portneuf River, near Inkom, Idaho. Crosby is part of a study that concluded bank erosion in the lower reach of Marsh Creek is responsible for heavy sediment loads, and establishment of wetlands in collaboration with willing landowners would be an effective way to improve water quality.

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POCATELLO, Idaho — Bank erosion in the lower reach of Marsh Creek is responsible for most of its excessive sediment loads, according to an Idaho State University researcher who has studied water quality in the impaired Portneuf River tributary.

Ben Crosby, an ISU associate professor of geosciences, said the findings could open new opportunities for landowners along the muddy stream to participate in cost-sharing conservation programs. Furthermore, Crosby said his research highlights how and where to target those conservation dollars, and clarifies that the source of erosion isn’t other tributaries, grazing in highlands near the creek’s headwaters, or low-land farm fields and pastures.

“The conservation action that would be most impactful right now would be construction of wetlands along the stream to slow water down and allow sediment deposition,” said Crosby, adding he’s discussed possible programs to stabilize banks and build wetlands with USDA’s Natural Resources Conservation Service.

In conjunction with Crosby’s study, Casey Taylor, a post-doctoral researcher within ISU’s Department of Political Science, will soon mail 1,500 surveys to landowners along Marsh Creek, which meanders through homes and farms and ranches of varying sizes upstream of Pocatello. Taylor said survey questions were based on previous interviews with 15 landowners along Marsh Creek and will seek to identify willing participants for future conservation efforts. Furthermore, she and her graduate students will ask landowners if they farm or ranch, how many cattle they may graze, whether they’ve participated in past conservation efforts and if they’re troubled by Marsh Creek water quality.

“Our goal is to produce a report that will help our conservation agency partners inform their future management strategies,” Taylor said. “One critical element that gets left out of a lot of studies of conservation projects is the perspective of landowners who are a critical piece of it.”

Inkom rancher Jim Guthrie Sr., for example, has planted about 2,000 willows along the creek through his property during the past 17 years, partnering with Farm Service Agency on some of his riparian improvements. Guthrie has also fenced his cattle out of the creek and added off-stream watering.

“I can’t believe how much (sediment) reduction there’s been,” Guthrie said.

Crosby explained efforts have been made to improve water quality in the stream for several decades. Though projects done over the years have helped to cut sediment loads in half, Crosby said Marsh Creek is still the major contributor of sediment into the Portneuf upstream of Pocatello.

Crosby and his students spent the past three seasons studying data from 13 sensors placed throughout the creek that transmit water-quality data every 15 minutes. They also floated the creek in kayaks to evaluate stream bank integrity.

Major funding for the study came from the National Science Foundation’s Managing Idaho’s Landscapes for Ecosystem Services grant. The City of Pocatello contributed $60,000 in seed money. Hannah Sanger, the city’s environmental coordinator, explained the city has to pay more for its permit to discharge stormwater because of the poor water quality in the Portneuf River. She said the city sees the greatest potential to improve river conditions by addressing issues in Marsh Creek.

“It’s pretty exciting to now have the data to see the options on the table that are going to be effective,” Sanger said. “That’s light years ahead of where we were.”



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