Spank, bj covered it exactly; the y-axis is height above sea level. Some people are just too dense to figure out what this means, so I'll explain as best I can:
According to
Wikipedia's Nile page, the primary source for the Nile originates at 8,858ft. Logically, the end is at 0ft, as it runs to the sea. Both of these
altitudes are relative to mean sea level, which is
a constant distance from the centre of the earth (well, almost, there are some variations due to the equatorial bulge, and local variances in gravity, but we'll use the simpler explanation for now). What does this mean? For every mile the Nile travels, it gets (on average) 2ft closer to the centre of the earth. To reiterate, this change is
not relative to a straight line, it is
relative to a curve. Is that really so hard to understand?? I don't get why so many people seem to struggle with something so simple...