Caddo Lake is a 25,400 acre (103 km²) lake and wetland located on the border between Texas and Louisiana, in northern Harrison County and southern Marion County in Texas and western Caddo Parish in Louisiana. The lake is named for the Southeastern culture of Native Americans called Caddo or Caddoans.

According to Caddo legend, the lake was formed by the 1811 New Madrid Earthquake. There may be some truth to the legend, as Reelfoot Lake in Tennessee was formed by that earthquake, but most geologists now feel that Caddo Lake was formed gradually rather than catastrophically. The lake was formed, either gradually or catastrophically, by the "Great Raft" a 100 mile (160 km) log jam on the Red River in Louisiana.

Caddo was first seen by Europeans in the 16th century, but substantial development would only begin with invention of the steamboat and US annexation of both Louisiana and Texas in the 19th century. The cities of Port Caddo, Swanson's Landing and Jefferson in Texas had thriving riverboat ports on the lake. Gradually as the log jams where removed in and the Red River by Captain Henry Miller Shreve and then by the Army Corps of Engineers, the lake changed shape and eventually fell over ten feet destroying the East Texas ports and their riverboat industry.

Industry once again came to Caddo Lake with the discovery of oil beneath it. The world's first over water oil well was completed in Caddo Lake in 1911. The Ferry Lake No. 1 was erected by Gulf Refining Company . The well bottomed at 2,185 feet and produced 450 barrels per day.

Oil derricks sprang up throughout the lake, around the turn of the 20th century, further damaging the fragile ecosystem. The oil industry left Caddo for richer fields at Kilgore and other locations in Texas. Texas tried to preserve parts of Caddo in 1934 by establishing a State Park, constructed by the WPA. The establishment of the Longhorn Army Ammunition Plant on the shores of Caddo, in the mid 20th century, polluted large portions of the surrounding wetlands until its closure in the 1990s.

In 1993 Caddo Lake preservation entered a renaissance, with the announcement that 7,000 acres (28 km²) of Caddo purchased by the Nature Conservancy were to be merged with the 483 acre (2 km²) Texas Caddo Lake State Park to be become the Caddo Lake State Park and Wildlife Management Area . In October 1993 Caddo Lake became one of thirteen areas in the United States protected by the Ramsar Treaty . As of 2003 Caddo Lake flora and fauna consisted of: 189 species of trees and shrubs, 75 grasses, 42 woody vines , 216 kinds of birds, 90 fish and reptiles, and 47 mammals. One of these species, Crataegus opaca or mayhaw fruit is collected from the water to make a jelly that is considered one of the finest in the world. Forty four of Caddo's native species were either endangered, threatened or rare . From 2001 until 2003 Caddo Lake residents fought a legal battle with the City of Marshall, Texas over water rights.

Cities on Caddo Lake

• Jefferson, Texas
• Karnack, Texas
• Mooringsport, Louisiana
• Oil City, Louisiana
• Uncertain, Texas