A brave COSI educator reached out to me recently, requiring my
assistance to settle a workplace debate. Brave because the educator
had a question for me that they felt was a little on the silly - or
perhaps worse, obvious - side, and didn't want to display their
apparent ignorance to the world. That natural reluctance, while
perfectly understandable, is a major impediment to science education,
and developing strategies to overcome that will be the subject of
But for today we have the central question: what direction do rivers
flow? North-to-south? South-to-north? Downhill? Something else?
Well, let's look at the major rivers of the world:
Amazon: west to east
Nile: south to north
Yangtze: west to east
Mississippi: north to south
Our own Scioto river flows from north to south, where it joins the
westward-flowing Ohio river, and eventually to the south-heading
So rivers definitely don't follow a particular direction around the
globe, but what sets their course? The answer is "downhill", but not
in the way you might expect. While the Amazon's headwaters are in the
mighty Andes mountains, the Mississippi is born in modestly hilly
To see why a river flows in a particular direction, you must ask
yourself: "If I were a drop of water falling from the sky, where would
I go?" If you fell on the eastern side of the Rocky Mountains, for
example, you might find your way to a small stream that connects to
the Missouri River, heading eastwards until you join the Mississippi.
Even though the Mississippi has humble origins, it's fed by rainwater
in the distant mountains.
The collection of all sources of a particular river is called its
watershed, and detailed mapping and sophisticated algorithms can
reveal what "downhill" really means to a river.