Nitixope, actually you are a little off on the source of the Clear Fork of the Trinity River. Due to my extensive cycling on the roads around North Texas, you will find that the town of Poolville is actually on the Clear Fork. The largest body of water near the source of the Clear Fork is Reservior Number Two, or Turpin Lake, west of Poolville. If you look on the aerials, you can even see a stream going further west. The source of the fork is north of E. Lone Star Rd. on Advance Rd. The vegetation associated with the stream stops at Turpin Lake Rd., north of E. Lone Star.
As for the separation between the Brazos River and the Clear Fork of the Trinity, there is a watershed, or ridge line running roughly along TX 171 between Weatherford and Cresson. To the southwest, the water drains to the Brazos, to the northeast, the water drains to the Clear Fork of the Trinity. Watersheds and drainage areas are shown on dfwmaps.com. You have to make a few clicks, but you can see where all the water is naturally flowing.