I do some gravel creek bed riding every once in a while. I also get to ride out on a sand bar when the Mississippi river runs low.
On any loose stuff, I would recommend a front tire with the center rib.
Holeshot GNCC
Maxxis Razr/ i-Razr
(I can't recommend Razr2, they create too much drag)
I would favor the i-Razr - minimal forward resistance (drag), center rib reduces sideways sliding of the front tires in a turn, also pretty tough.
I've tried a couple different tires on the back:
stock Raptor 660 - they bite down better than the stock 700's
Maxxis i-Razr - my personal favorite - those corner knobs create a slight paddle action for going forward and really dig in a turn (just ask any of my friends who have lost sight of me behind the wall of roost I've left on them)
Pure Sport Realtors - a reversible tire for digging when mounted one way, and scooping (paddling) when mounted the other way. They'll vibrate on hard surfaces, but they REALLY hook up on loose stuff. Forward traction is their main feature, they can make it pretty tricky to turn sometimes. They're kinda on the heavy side but you can run them with 0 pressure for a really wide contact patch. They're worth looking into, although they might be harder to find. I don't think Pure Sport makes them anymore, but I have seen them listed in Titan's line-up.
You can also look into ITP's Mudlite SP. I have no experience with them, but they look like they'll do some serious digging!
