LiveOak said:
Park somewhere legally, or get a drop off so you don't have to sweat your vehicle getting towed.
Then just stay in the river bed and tell land owners to pound sand.
No. Here is why.
I was recently at the Narrows, legally, having been escorted there by a ranch manager because I'm doing some business with the owners and was asked if I'd like to see it.
The ranch manager told me multiple stories about dozens of trespassers parking ahd hiking across the adjoining ranches, some of whom were in very bad shape from dehydration etc by the time they were found. Some had no clue that they were on private property, or at least acted like they didn't, and some were blantanly trespassing and didn't care.
The ranch manager has asked multiple people if they had enough water for the hike, keep in mind that you are looking at roughly a 14 mile round trip and it is not a straight shot. There is very little shade and there are no ammenities nearby. Now, this ranch manager is a nice guy, but telling him to pound sand isn't smart since he's had to rescue multiple people, usually from Austin that had no clue what they were really doing.
Legally speaking, if you stay in the center of the river you are technically legal. The locals may check on you but there is a reason for that. The manager told me his biggest fear is finding a body. This same manager has rescued the same people that told him to go "eff" himself, when he checked on them.
So, you might try being polite and see how that works. Also, I do not know of any legal parking anywhere you can get to or from the Narrows, so you may have to be dropped off.