Marked Tree Lock and Siphons, Flood control facility in Poinsett County, United States

Marked Tree Lock and Siphons is a flood control facility on the St. Francis River in Poinsett County, Arkansas. The 1926 lock sits on a disused artificial channel west of the river, with a concrete trough about 130 feet long and 30 feet high and a small brick control house. The 1939 siphons are three steel tubes, each nearly 70 meters long and 2.75 meters in diameter, mounted on supports across a levee about 200 meters northeast of the lock.

The facility was built in 1939 by the Memphis District Corps of Engineers to serve Drainage District Number Seven and protect the region from flooding. This project represented an important step in how communities managed water and defended themselves against seasonal river overflow.

The name comes from the nearby town of Marked Tree, not from a surviving tree. The lock and siphons were built for Drainage District Number Seven, and the siphons were dedicated on 7 June 1939. Engineers from Britain, France and South America visited them, according to the U.S. Army Corps of Engineers.

The facility sits along the St. Francis River on a levee and is accessible for visitors to view the engineering structures from different vantage points. Visiting in dry seasons makes it easier to observe how the system works and understand the overall layout.

The siphon design was borrowed from New Orleans, where water mains crossed levees by siphon. The Army Corps of Engineers could not find siphon experts through research, yet later tests showed efficiency normally achieved only in laboratories. Engineers from Britain, France and South America visited the site.