Metzler Park
Green River Offroad
The first part of the trail to the river is wide and smooth.
Soon you come to a long boardwalk over some marshes.
  It is a tight squeeze for a trike.
Soon the trail gets muddy, but a bed of river rock keeps you out of it.  Suspension would have helped.
As the track closes in on you, the rocks turn to sand, and pedaling becomes more difficult,
and then is impassable.
The trail contains several forks.  When the first one dead-ended at a side channel rather than the river, I tried another with the same result.
I never found the Green River on that trip, but discovered this singletrack on another,
and there it was.