Chords posted so far just aren't quite right.
It's mainly good old chord I (Em) and V (B7)
The Em/B is optional as the chord inversion comes from the bass playing a B in the 
but playing solo it sounds 'right' without the low E sounding.

This is my first tab on the web - so be easy on me!

Capo on first fret

       E-A-D-G-B-E
  Em = 0-2-2-0-0-0----------|
Em/B = x-2-2-0-0-0----------|
  B7 = x-2-1-2-0-2----------|
  G  = 3-2-0-0-0-3----------|
  C  = x-3-2-0-1-0----------|
  D  = x-x-0-2-3-2----------|
B7/F#= 2-2-4-2-4-2-|


 Em        B7/F#         G       Em/B
I've got a mule, and her name is Sal,
Em/B                 B7   Em
Fifteen miles on the Erie Canal.
(Em)    Em       B7/F#        G        Em/B
She's a good ol' worker an' a good ol' pal,
Em/B                 B7   Em
Fifteen miles on the Erie Canal.
      G                  D
We've hauled some barges in our day,
 Am                  C
Filled with lumber, coal, and hay,
 Em         B7/F# G           Em/B
And we know every inch of the way
(Em/B)         B7   Em
From Albany to Buffalo.

 G           D
Low bridge, everybody down!
 G           D                       Em
Low bridge, for we're comin' through a town!
(Em)        Em                B7/F#
And you'll always know your neighbor,
          G
You'll always know your pal,
 B7                                    Em
If you've ever navigated on the Erie Canal.

     Em           B7/F#       G        Em/B
We'd better look around for a job, ol' gal,
(Em/B)               B7   Em
Fifteen miles on the Erie Canal!
           Em       B7/F#    G               Em/B
'Cause you bet your life I'd never part with Sal,
(Em/B)               B7   Em
Fifteen miles on the Erie Canal!
 G                      D
Git up there, mule, here comes a lock,
 Am                  C
We'll make Rome 'bout six o'clock,
 Em      B7/F#    G          Em/B
One more trip and back we'll go
(Em/B)             B7   Em
Right back home to Buffalo.

 G           D
Low bridge, everybody down!
 G           D                       Em
Low bridge, for we're comin' through a town!
          Em                B7/F#
And you'll always know your neighbor,
          G             Em/B
You'll always know your pal,
(Em/B)                          B7   Em
If you've ever navigated on the Erie Canal.

etc etc

any corrections mail me at travellinjacks at yahoo dot co uk