when you turn on greenbelt off of greenoaks there is a sharp right hand turn at the water treatment plant. As soon as you turn right there is a field on the left with no gate if you go through that gate and head straight back you will run into the river there is an open spot in the trees that with a rope you could slide your canoe/yak down to a flat spot jsut below the water discharge and float down to the bridge.
To make it easy for y'all, here's a satellite view:
http://maps.google.com/maps?f=q&hl=en&geocode=&time=&date=&ttype=&q=greenbelt,+arlington,+tx&sll=37.0625,-95.677068&sspn=49.844639,60.820313&ie=UTF8&ll=32.783955,-97.139697&spn=0.013025,0.014849&t=h&z=16&iwloc=addr&om=1
Field he's talking about is on the left of where Greenbelt turns north.
Here's a closeup view of the bridge itself:
http://maps.google.com/maps?f=q&hl=en&geocode=&time=&date=&ttype=&q=greenbelt,+arlington,+tx&sll=37.0625,-95.677068&sspn=49.844639,60.820313&ie=UTF8&ll=32.788086,-97.139965&spn=0.003256,0.003712&t=h&z=18&om=1
There's actually a place on the west side of the road just south of the bridge where 4WD trucks have been pulling off, and there's a kind of path down to the bank. It's still steep down to the water, but not as high as the bank in the areas nearby, so this is doable, but I would bring a rope.
I saw a homeless guy walking along there and gave him a ride, and he said he wouldn't leave a vehicle there (for what that's worth). I told him I'd leave my bull mastiff in the vehicle, but nobody here has reported any problems anyway.
By the way, this is also about 1/2 mile to a mile south of Trinity Blvd, which may make it easier to find for some people. I work at Bell Helicopter on Rt 10 in Hurst, and this is just a short distance from there.