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Originally Posted by GH85Carrera

ARKANSAS: The Marked Tree Siphons installation, dating to 1939, is said to be the only engineering structure of its kind in the United States. Built at a cost of $215,000, the three steel tubes integral to the design were among the largest in the world, with diameters of nine feet and lengths of 228 feet. As described by the Arkansas encyclopedia, "Their operation was deceptively simple. First, a vacuum pump primed each siphon. Once the siphon was primed, the vacuum pump was turned off, and the flow was self-sustaining. An air valve regulated the rate of flow."
Once activated, the siphons can run indefinitely, powered only by gravity. Eight decades later, the pumping process still functions properly after an overhaul in the 1990s.

Lots of good info here
https://encyclopediaofarkansas.net/entries/marked-tree-siphons-3999/
Just a few excerpts

The siphons were also a unique application of an engineering structure of their type, designed to lift the flow of the St. Francis River over an earthen levee and deposit it in the river channel on the other side of the levee.

In December 1937, the Corps of Engineers began repairs to the sluiceway. On May 7, 1938, high waters washed out a ninety-foot gap in the levee. The sluiceway settled further and was damaged beyond repair. By October, the levee break was repaired and the sluiceway removed.

According to the Corps of Engineers, the sluiceway and levee failed because they were constructed on a stratum of fine sands that tended to become “quick” when saturated. Repair or replacement of the sluiceway was deemed too costly, so the Memphis District announced it would permanently dam the levee gap, with the plan that “instead of passing water under or through the dam, water will be siphoned over it.”

The Marked Tree Siphons
[IMG]These three gigantic siphons, each nine feet across and two hundred feet long, were built in 1939 for $215,000 to take water out of the St. Francis Lake, also known as the St. Francis Sunken Lands, hoist it over the levee and dump it into the St. Francis River (on the right side of the picture below) in order to keep the river navigable. Normally, that kind of thing is done with culverts, locks or sluiceways; but the ground here is so soft and sandy that those usual solutions create weak points in the levee. The siphons allow the levee to maintain its structural integrity while simultaneously allowing engineers to shift water from one side to the other. I spoke with Danny Max of the Memphis District of the Corps of Engineers, and he told me that he knew of no other such siphons on the whole planet. Sunken Lands on the left, St. Francis River on the rightThe most elegant thing about these siphons is the clever way that the flow is regulated. There are no water pumps and no moving internal valves or constrictions. Once the siphons are primed by vacuum pump, the flow can be constricted by opening an air valve introducing a bubble into the top of the arch. Once primed, the siphons can run indefinitely powered only by gravity and the flow can be adjusted with a twist of the wrist. The City of Marked Tree is named for a tree which was blazed with an "M" which marked a portage point between the St. Francis and Little River. The two rivers, which run in opposite directions, come within a quarter mile of each other here at Marked Tree. If you're paddling up the St. Francis, you can drag your boat the quarter mile overland to the Little River and go downstream to the confluence, saving yourself about eight miles of fighting the current. There are two stories about the mark on the tree. One holds that it was put there by the infamous frontier bandits of the John Murrel Gang (thus the "M"), and another holds that it was first marked by Indians. Actually, both stories could be true. The tree washed away in the flood of 1890. Incidentally the St. Francis Sunken Lands are sunken because they dropped fifty feet in elevation during the New Madrid earthquakes, the first of which, on December 16, 1811, is estimated to have been 8+ on the Richter scale making it the most powerful quake in recorded history.[/IMG]