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.
The 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.
To visit the siphons, take highway 63 north from West Memphis. When you get to Marked Tree, look for the brown sign saying "Siphon Access." Drive on the dirt road for about five miles and there it is.