Bob,
I actually chickened out in that spot, but I'm building a bridge in another. I'm using 25' Tamerack trees-12" at base, and 8" at 25'. I'm placing 2 on the left, 1 in the middle, and 2 on the right. They will lay on 2 railroad ties on both ends. The 2 tameracks on either side will will be bound together by cable, and I have some creosote treated 4x6's in 5' to 7' lengths that will be staggered accross the bridge to build a base. Then treated planks will be used on both sides-over the outside logs, for runners.
The center of the connected outside trees will be 6'-the width of the tractor. The middle trunk offers further support, and nothing will be actually be sunk into the creek, or bank. The actual creek width is around 13', so a little more than 5' will cover the bank on each side.
This is how the loggers build their bridges in the woods, they are considered "temporary". Most of the loggers don't have the luxury of using Tameracks, and they use cedar posts and dirt as a base.
The logger that completes my dozer work helped me design the bridge, and he offered to take his dozer over first to make sure of the strength. Nice guy!!
Jeff...U.P. of Michigan.