Great 'splaination'! I just came back from outdoor school with son FiveOSix's class and one instructor explained similar, but this elevation map rocks it!Well, lucky for you, MDS has a resident Geographer on staff.
Here let me "splain it to you"
Contour lines show the height above sea level, also known as "Elevation".
The first picture shows the river not to far from it's origin. Notice the elevation is roughly "2400".
Now take a look at the second picture, where the river is close to where it enters the lake in PA. The elevation is roughly 1500. That is a 900 foot drop. That is why it runs "north".
Thanks for posting it up.