Jon,
Here is quick read taken from an article I found. It provides a real nice explanation.
Quoted:
Creek crossings generally occur at the center of an “S” curve in the creek. The bends of the “S” are generally holes with high banks on the outside, while the straight portion between the bends is generally shallower with a lower bank. That is where the deer cross.
Some creeks don’t meander, making it much harder to find crossings without actually walking the banks of the creek to look for cross-trails.
Ditch crossings typically take three forms. Either the deer go around the ditch at the top or bottom (comprising the first two forms) or they cross the ditch itself at a place where the banks are gradual. The first two crossings are easy to find from a map study, but you have to walk the ground to find the third type.