Texas Almanac Q & A

Bob Ray Sanders, up the Trintiy River basin way in Fort Worth where they print the Star-Telegram newspaper, composes for his column a quiz using the latest Texas Almanac (now published by the Texas State Historical Association).   The questions are at http://www.star-telegram.com/news/columnists/bob_ray_sanders/story/1881243.html The answers are at http://www.star-telegram.com/news/columnists/bob_ray_sanders/story/1898783.html Question # 2 asks about the longest river "within" (quotation marks in original work)  Texas.  Well, I knew that the "within" would exclude the Rio Grande and Red River. So I thought it was the Brazos, and may still contend so.  Sanders states the answer as the Colorado River, a fine river, but not the equal to the Brazos in my estimation.  I'm guessing that Sanders is working on the presumption that the upper feeders of the Brazos carry water often enough to not be completely "within" the Texas-New Mexico boundary.  On the other hand the upper feeders of the Colorado rarely carry water and hence are not worthy of being a true part of the Colorado River basin and hence is "within" Texas?  Then again maybe the measurement is different by crow-flight or river-bed.  Where's a good hydrologist when you need one?  Maybe I should just get a new Almanac!