Which river forms part of Texas's eastern boundary?

• A. Rio Grande
• B. Sabine River
• C. Nueces River
• D. Red River

Answer: (B)

The river that forms part of Texas’s eastern boundary runs along the Texas–Louisiana line in the northeast, creating a natural border between the two states. This is the Sabine River, which stretches from near the eastern edge of Texas down to Sabine Lake before meeting the Gulf of Mexico. Its position at the state’s edge on the east is why it’s the river that forms part of Texas’s boundary.

The other rivers don’t serve as that eastern boundary: the Rio Grande marks Texas’s western boundary with Mexico; the Nueces flows through southern Texas but does not separate Texas from another state; and the Red River forms the northern boundary with Oklahoma.