Sorry I missed this one earlier. You mentioned Brazos & Navasota, as well as TX 21 & FM 60 but did not clarify. FM 60 would only be Brazos. Did you also mean the Brazon Tx 21 also? If not, I can tell you a little from my past experiences from 1980 thru 1993.
From TX 21 bridge ... if you go upstream ... bend to the right, the slow long bend to the left, ravine or creek feeds in about midways around the bend to your outside. Randy caught a 19 # Yellow on a willow limb line there with a big fat minnow (1982). As you clear that bend, a shoal ledge on your right, then eventually a cliff Rockwall on your left. Used to be a shelf ledge about a 3rd of the way upriver towards the left (down under). But it seemed to have been knocked out (down) when I camped there for over a week on the sand bar in 1993. A lot of Yellows were caught there in 1980 by Newt- biggest #5#. summer 1981- me a 28#. And Randy a 22 or 24# in 1982. If you go further up, a bend to your right, then a bend to the left. There was some sort of camp there back in 1982. There are either Bluff, Shoal or Rockwalls up little bit on your left side. Seemed to be some nice holes there. However, we sheared the prop going thru the gravel bed between bends between both Wall areas.
Going down from the bridge, if you do not count the "jut", it is a long ways down to the 90 degree bend to your left. A little further & the next bend is to your right. Just on your left before that bend, is where the Little Brazos River feeds into the Brazos (Big Brazos) River. If I recall, there might have been some rapids & rocks in the bend there.
Also, backing up on TX 21 to the Little Brazos River, if you go upstream, there is a portion they call The Blue Hole. I've never been up there, but others said it was good fishing there. Not sure how far upstream it is.
Now, as to Navasosota River the two names you mentioned I think I have heard, but am not sure if either one is Tx 21 or not. There is Ferguson Crossing (on TX 30), Democrat Crossing (Democrat Road off the Kurten ByPass now), and off the OSR. I forget that one's name, but my Dad & his fishing buddy fished it well in late 50s & early 60s. Including with P&G soap. Another crossing was down FM 2038 from Kurten, that goes past the back side of Reliance & eventually the pavement ended. The 3rd bridge (trestle) was the River. That was back in the 1970s. And I forget the one crossing's name on TX 7 just up from Marque about 5 miles.
If you have not tried Tx 21, the 3rd bridge (heading to Madisonville) is the River. If you take it up aways, you hit a Tee, upstream gets fairly wide here for awhile. Biggest one in this area was a 26# Op- April 1999. If you straight at the Tee, it will take you to the 2nd bridge. If you head up the second bridge towards the Tee, there are a few good holes. I never got to fish them good, as I had discovered their depth by accident scoping out possible holes. If you go downstrem from 2nd bridge, some good Blues are in this area from the bridge to the old dead tree as it narrows. There is a deep hole down aways. I'll let you find it. About 3 or so bends past it, it opens into a very wide area. Fairly deep here.
Also, next time I will touch on Black's Bridge off from Calvert on the Brazos River. It was either 1 or two blocks past the Dairy Queen that you turn left on a FM road. About 8 miles from Calvert.