Louisiana v. Mississippi

Background

The dispute involves the State of Louisiana, the State of Mississippi, and a Mississippi landowner, Avery B. Dille Jr. In 1970–1971 both States made leases covering the same stretch of riverbed. An operator drilled a directional oil well from Dille’s land on the Mississippi side; the well was completed in January 1972 and has produced continuously. Dille sued in 1979 claiming the river boundary had moved so the well’s bottom hole was in Mississippi. Louisiana began this original action asking the Court to decide whether the bottom hole lay in Louisiana or Mississippi.

Reasoning

The Court focused on where the “live thalweg” — the usual downstream route followed by river traffic, used here to mark the state line — ran relative to the well for 1972–1982. A Special Master held a trial, and three expert engineers relied on U.S. Army Corps hydrographic surveys, Coast Guard sailing recommendations, and navigation aids. Louisiana’s experts placed the sailing line east of the well each year; Mississippi’s expert placed a moving line that sometimes crossed over the well. The Master found the Louisiana experts’ interpretation better matched the surveys and navigational practice for the disputed years. After an independent review, the Court confirmed the Master’s findings.

Real world impact

The ruling determines that the well’s bottom hole was within Louisiana for 1972–1982, settling who controls the leases and the well’s production for that period. The Court declined to map a more detailed multi‑year boundary for the whole reach. If the river’s channel shifts later, either State may bring new litigation.