Here’s an in-depth look at sedimentation and U.S. reservoirs from H2O Radio. Click through and listen to the whole show. Here’s an excerpt from the transcript:
Sedimentation occurs when all the sand, silt, rocks, and soils that would naturally travel down rivers to the sea get trapped behind dams. Sediment itself is a good thing. It creates habitat, fertile farm fields, and forms deltas at the river’s mouth that are natural buffers against storms in places like Louisiana. But when dams were built along the Missouri River, that natural process came to an end and the troubles began.
Perhaps nowhere is sedimentation more evident than at the confluence of the Niobrara and Missouri Rivers, where the water snakes its way through lumps of grass-covered islands. The sand has piled up there ever since the completion of the Gavins Point Dam just downstream. When the sediment-laden river hits the deep standing water of its reservoir, called Lewis and Clark Lake, the water loses its energy and the load it’s carrying drops out.
Over time, the sediment mounds up to form a delta and the river is forced to go over, around, or even under it. That happens rain or shine, but during a major flood disaster, like the recent bomb cyclone, levels are pushed up proportionately, if not higher—a point the catastrophic flooding from the Spencer Dam failure made all too clear, says Nicholas Pinter, professor of earth and planetary sciences at the University of California, Davis. He says that sedimentation caused by Gavins Point Dam made the destruction of the Janak’s property more likely—“absolutely, unquestionably—and no honest person in the Corps or otherwise would say not.”
The “Corps” he’s referring to is the Army Corps of Engineers. They built Gavins Point Dam in 1955, but in the 1970s they had to move the town of Niobrara because sedimentation resulting from the dam’s reservoir was raising the water table, causing flooded basements and ruined crops.
Actually, the town of Niobrara had to be physically moved—twice. The first time was in 1881 when an ice jam on the Missouri flooded the village with nearly six feet of water. The townsfolk pulled its buildings by oxen and mule a mile-and-a-half to higher ground where they thought they would be safe. But that was before Gavins Point Dam was built.
No one disputes that the reservoir was responsible for the high groundwater in Niobrara in the 1970s. In fact, the engineers who built the dams in the 1940s and ’50s along the Missouri River knew sedimentation would be a consequence. But, says Pinter, Gavins Point Dam was “designed as almost every large dam on Earth, which is not to say that it was well designed or poorly designed, but it was well known at the time that it would not pass the sediment that it would need to, to avoid this problem.”
The fact that the dams would not pass sediment—and that eventually sedimentation would make them inoperable—was not only fully acknowledged by the engineers who designed them, but they even had a name for it—the “Sediment Design Life.” As Tim Randle, Manager of the Sedimentation and River Hydraulics Group at the Bureau of Reclamation explains, “Virtually all reservoirs in the U.S. and much the world were designed with a sediment design life, meaning after so much time the reservoir’s not going to function very well, the outlet will be plugged.” For Reclamation and Army Corps dams, he says, that lifespan was somewhere around 100 years.