Here is the MVUM for the Angelina NF. It is the legal controlling document for motorized travel on the Forest, a requirement of the 2005 Travel Management Planning Rule. Among other things the 2005 Rule via a public process:
1) Designated Motorized Routes
2) Prohibited motorized use other on Designated Routes (ie no cross county travel, no user/club trails)
3) Required publication of a Motor Vehicle Use Map
A quick look, the MVUM seems to indicate there are no Designated Motorized MC trails in the Angelina. The routes shown being "Roads Open to Highway Legal Vehicles", some having a seasonal closure.
It is extremely unfortunate that in many cases, on many forests, OHV users were all too often not fully engaged during Travel Management Planning. Many thought the FS staff would provide for OHV recreation, but often the enviros were much better organized and represented. Their voices were heard and often resulted in Forest Plans with greatly reduced or even no singletrack motorized trails. I have heard the that once Travel Management was completed for all the NFs in Arizona, not a single MC trail had survived (MC groups are work hard to correct the oversight).
In your case the MVUM shows no Designated OHV routes and as a result non designated trails would be viewed as cross county travel and therefore illegal regardless of streams or roots.
Here is a link to the FS web page with links to the MVUMs for other Texas Forests:
Sorry to be the bearer of bad news but perhap during the next Planning cycle the OHV community can/will press for OHV recreation opportunities. Sucks, I know.