Shaneystar's suggestion is a good one, since the Brazos is a major river in Texas... but the answer to your question is most likely the Sabine since it not only forms part of the boundary between the States of Texas and Louisiana (north to south) but was also the International Boundary with Mexico in the early 1800's...
I would have thought that in the 1830's they would have been referring to the two big known main rivers The Missouri and The Mississippi. Although someway from Texas, there were no states west of this line. If you were going west to settle you either went to Texas or Indian country.