LALouisiana Geography

Near Simmesport, a concrete structure decides where the Mississippi River goes. Left alone, the river would already have abandoned its lower channel and jumped west into the Atchafalaya, which offers a shorter and much steeper route to the Gulf. That would strand Baton Rouge and New Orleans on a tidal creek. The Army Corps of Engineers finished the Old River Control Structure in 1963 to hold the split at roughly seventy percent down the Mississippi and thirty percent down the Atchafalaya, freezing the 1950s distribution in place. The 1973 flood scoured out the foundation of the low sill and came close to taking the whole thing out.

A ceiling of 535 feet and a floor below the sea

The highest ground in the state is Driskill Mountain in Bienville Parish, a wooded hill of 535 feet. Only Florida and Delaware have lower high points. The lowest ground is in New Orleans, about eight feet below sea level, so the entire vertical range of Louisiana is around 543 feet, one of the narrowest of any state. About half of New Orleans sits at or under sea level, and it has been sinking since the swamps behind it were drained in the early 1900s and the peaty soil began to compact.

The elevation Louisiana does have arrives from an unexpected direction. Deep underground lies the Louann Salt, a Jurassic evaporite bed that is less dense than the sediment stacked on top of it and slowly squeezes upward in columns. Where those columns reach the surface they lift the marsh into the Five Islands: Avery, Weeks, Cote Blanche, Jefferson and Belle Isle. Avery Island rises about 163 feet out of flat wetland, holds a working salt mine and grows the peppers used for Tabasco.

Salt geology also produced the strangest single day in Louisiana's physical history. On November 20, 1980, a drilling rig on Lake Peigneur punched into the Diamond Crystal salt mine underneath. The lake, about 1,300 acres and ten feet deep, drained into the mine through the hole. The Delcambre Canal reversed direction and poured in from the Gulf, briefly producing the largest waterfall the state has ever had. Nobody died, and the lake refilled as a deep saltwater body.

The coast that is measurably leaving

Louisiana exists because the Mississippi spent seven thousand years dumping sediment across the northern Gulf, switching channels every thousand years or so and building a new delta lobe each time. The modern bird's foot delta below Venice is the youngest. Levees now hold the river inside its banks all the way to Head of Passes, so the sediment shoots off the edge of the continental shelf into deep water instead of spreading across the marsh, and the marsh no longer gets rebuilt.

The consequence is measurable rather than theoretical. USGS mapping puts the loss at close to 2,000 square miles of coastal land between 1932 and 2016, from a combination of sediment starvation, subsidence, saltwater intrusion and canals cut for oil and gas access. Isle de Jean Charles has shrunk to a fraction of its 1950s footprint and its community has been relocating inland.

What remains is still enormous. The Atchafalaya Basin covers about 1.4 million acres as the largest river swamp in the United States, a cypress and tupelo floodplain that still behaves as a working delta because it is the one place still receiving sediment. Lake Pontchartrain, 630 square miles of brackish estuary rather than a true lake, connects to the Gulf through the Rigolets and Chef Menteur Pass; the causeway across it runs 23.83 miles. Louisiana's general coastline measures only 397 miles in a straight-ish line, but trace every bay, bayou mouth and barrier island and the tidal shoreline runs to roughly 7,700 miles, behind only Alaska and Florida.

Borders drawn by a treaty, a parallel and a two-month republic

The western boundary is the Sabine River, and it was contested long enough to create a lawless zone. Between 1806 and 1821 the strip between the Sabine and the Arroyo Hondo was the Neutral Ground, agreed by Spanish and American commanders as territory neither would police, which is exactly how it was used. The Adams-Onis Treaty of 1819 finally fixed the Sabine as the line. Today the river is dammed into Toledo Bend Reservoir, about 185,000 acres and the largest reservoir in the South, shared with Texas.

The northern boundary with Arkansas is the 33rd parallel, a surveyed line with no relationship to anything on the ground. The eastern boundary runs up the Mississippi and then along the Pearl River, and the reason the state has a toe east of the Mississippi at all is a piece of political improvisation. The Florida Parishes were part of Spanish West Florida, not the Louisiana Purchase. In September 1810 residents seized the fort at Baton Rouge and declared the Republic of West Florida, which lasted about seventy-four days before the United States annexed it. Those eight parishes were attached to Louisiana at statehood in 1812. Louisiana counts them as parishes rather than counties, the only state to use the term, inherited from the ecclesiastical divisions of the French and Spanish administrations and written into law in 1807.

Heat, rain and the storm record

The record high is 114 degrees at Plain Dealing on August 10, 1936, and the record low is 16 below zero at Minden on February 13, 1899. Rainfall averages between 50 and 65 inches a year, rising toward the southeast, making Louisiana one of the wettest states. The whole state observes Central Time.

Hurricanes arrive on a shoreline with almost nothing to slow them. Katrina came ashore on August 29, 2005 and the levee failures that followed flooded about eighty percent of New Orleans. Rita hit the southwest coast weeks later. Laura made landfall near Cameron on August 27, 2020 with 150 mile per hour winds, and Ida struck Port Fourchon on August 29, 2021 at the same intensity, sixteen years to the day after Katrina.

Neighboring states and more Louisiana pages