Flooding changes the course of the Mendenhall River near Brotherhood Bridge

Just before Brotherhood Bridge, the Mendenhall River takes a short detour. It loops east for a few hundred yards and then doubles back. It’s called an oxbow, a U-shaped departure from the river’s trajectory.

But it might not have this shape for long. The extra water in the river from flooding cut through the oxbow Thursday.

Tom Mattice is the city’s emergency program manager. He said this spot on the river has been eroding for years.

“It went from a trickle of water to 100 yards wide in about an hour,” he said. “And as that continued to widen, the trees on the banks continued to fall into the hole, and as they would block the hole, the water would go to the outsides around the trees and continue to erode the bank further and further.”

Water flows through the new gap in the oxbow on Mendenhall River. (Photo by David Purdy/KTOO)

The river hasn’t completely changed course yet — there’s still water flowing around the old route. But Mattice thinks that will change over time, possibly during future flooding. Exactly what those changes will look like or when they happen is hard to predict.

But he said anyone with property along the river should count on erosion.

“If you don’t stabilize your banks, your banks are going to change,” he said. “And so, this area will have probably more change than some other areas, but everywhere on the river has change.”

Water flows through the new gap in the oxbow on Mendenhall River on Thursday. (Photo by David Purdy/KTOO)

Mattice said the river is probably changing faster than it has in the past because 11- and 12-foot floods are more common now.

“If you have one 100-year flood in an area, then the trees grow back,” he said, “and the next time that event occurs the trees are 99 years old protecting that bank. When you have a 100-year flood back to back to back to back to back, the trees and vegetation can’t keep up. So in general, due to this accelerated change on the Mendenhall, due to a lot of high water events, the river banks are probably less stable than they are traditionally.”

He said sooner or later property owners along the river will have to stabilize their banks, or see them worn away by the river’s natural tendency to change course over time.

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