Question: What is the longest river entirely in the state of Texas?
Answer: Either the Brazos River or the Trinity River, depending on the parameters. The Brazos River begins in Stonewall County at the confluence of the Double Mountain and Salt forks, flowing 938 miles through Waco and the Coastal Plains to Freeport. Ninety-four percent of its 45,510-square-mile watershed is in Texas, while 6% is in New Mexico.
On the other hand, 100% of the 423-mile Trinity River’s 17,969-square-mile drainage basin is in Texas. It begins at the confluence of the Elm and West forks in Dallas County and flows through Southeast Texas, emptying into the Gulf near Anahuac.
