Animaterra, thanks for the site link which gives explanation of several lines in the song no one has mentioned.
1) When the sun goes back and the first quail calls
Follow the drinking gourd
This advises the escaping slave to leave in late winter. The assumed time of travel was one year from the area of Central and South Alabama ,where this song was known, to the Ohio River, which was the major obstacle. This would put the slaves at the Ohio in winter when it was frozen and crossable.
2)The river bed makes a mighty fine road( the Tombigbee River in Central Alabama)
3)The river ends between two hills(headwaters of the Tombigbee)
There's another river on the other side(the Tennessee River)
4)Where the little river meets the great big one(where the Tennessee runs into the Ohio)
The Old Man's waiting to carry you to freedom(Peg Leg Joe)
Thanks again, and especially to Pablo for starting this one.