Some natural borders en México

RIOS

Flowing landmarks that separate regions and are vital to Mexico’s geography.

The clue refers to natural borders in Mexico, which often include rivers dividing regions or marking boundaries between states. "Ríos" is the Spanish word for "rivers," a common geographic feature that serves this purpose, like the Río Bravo along the US-Mexico border. The answer plays on the idea of natural land divisions, making it a straightforward but culturally relevant hint.