You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
Q: Category: THE 19th CENTURY 
Clue: Under  the 1848 treaty of Guadalupe Hidalgo, this river became the boundary between Mexico & Texas
A: 
the rio grande