[iainmerrick]

water and gravity have formed the surface to have few local minima

From the article: "It turns out that the earth is filled with local minima"

(I suspect the deal is that you're right that water always flows downhill to the ocean; but that it often happens at scales too small to capture in the GIS data, or underground.)

[chrisfosterelli]

Yeah you got it! Water moves in ways that are not captured by the resolution of the elevation dataset. Also, water moves into/through the ground.