John Paul Hammerschmidt Lake is a reservoir on the Arkansas River and an integral part of the McClellan–Kerr Arkansas River Navigation System (MKARNS).

It was formed by constructing the James W. Trimble Lock & Dam 13 across the river, near the city of Barling, Arkansas and extends upriver 26 miles (42 km) to W. D. Mayo Lock and Dam, which is located inside the state of Oklahoma.

The Arkansas-Oklahoma State Line is the western boundary of both Arkansas counties.

[1] Poteau River, an Arkansas River tributary which flows north in Oklahoma, crosses the Arkansas-Oklahoma state line immediately southwest of Fort Smith and discharges into Hammersmith Lake.

The normal elevation of the lake is 392 feet (119 m) above sea level.