She digs up the dirt

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"We need to figure out what the mud is telling us." (Photo by Dan Engstrom/Courtesy SCWRS)
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Laura Triplett, a grad student at the University of Minnesota, and Dr. Mark Edlund of the Science Museum's St. Croix Watershed Research Station, prepare to take a sample of mud from the bottom of Lake St. Croix. (Photo by Dan Engstrom/Courtesy SCWRS)
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Triplett cuts a sediment core into sections. Analyzing these will tell her the age and composition of the mud. (Photo by Mark Edlund/Courtesy SCWRS)

Laura Triplett is uncovering the history of the St. Croix River-literally. Working at the Science Museum's St. Croix Watershed Research Station, she digs up samples of mud from the bottom of Lake St. Croix and analyzes them for clues to the river's past.

"To protect a place you care about, you need to understand its history," Triplett says. Though logging, farming and cities have been affecting the St. Croix for over 150 years, scientists have only studied it since the 1970s. To learn how the river has changed over longer periods of time, Triplett examines the mud that settled on the lake bottom in decades past.

The river's history is preserved in the mud

Fast-moving river water carries sediment with it. When the river hits a wide valley, like Lake St. Croix south of Stillwater, the water slows and the sediment settles to the bottom, preserving a record of river conditions.

Triplett drills into the lake bottom and removes long tubes of mud, known as cores. Studying mud of different ages, she found that, surprisingly, large-scale logging in the 1850s did not alter the river very much. However, there was a huge increase in both the amount of mud and the phosphorus it contained beginning in the 1940s.

Figuring out what caused that change will help us protect and restore the river.

Lab work unlocks the river's secrets

At her lab, Triplett puts the mud samples through several tests:

  • She determines the age of the sample by measuring the lead in it. (Small amounts of lead occur naturally in river water, and over time changes from one form to another. Measuring how much change has occurred gives a very accurate age for the sample.)
  • She measures the amount of mud at different periods, and calculates how much accumulated at that time. (High accumulation means high rates of erosion.)
  • She also measures organic matter-the remains of plants and animals. This tells her how much life the river supported.

Her colleague Mark Edlund looks at samples under a microscope, counting and identifying algae. Together, this information gives them a history of the river.