And the answer is, this:
It's smaller than a valley, wider than a ravine, deeper and longer than a draw. In our area they have been carved by creeks into the flat plain left behind after the massive glacial sheets of ice retreated back to the mountains at the end of the last ice age. Since the word is of French origin, I assume we have the early French Canadian trappers to thank for its prevalence, given that they were the first white men to venture into this area.
So now you now. It's not a gulch, a gulley or a gorge. It's a coulee.