An assessment of ancient drainage systems on Earth, Mars and Titan provides new insights into the topography-generating mechanisms on planetary bodies. The results illustrate the diverse geological processes affecting these bodies and their river networks. The movement of liquids such as water across a planetary surface forms rivers, signatures of which can be seen even long after the liquid has disappeared - as with ancient riverbeds formed by water on Mars, for example. Studying these systems can provide valuable insights into the long-term evolution of a planetary body's topography, as the layout of the drainage network depends on whether it formed before, after or at the same time as the topology. Planetary processes that affect the entire body, such as thermal expansion and contraction, can create "long-wavelength" topographic features that river flow directions will typically correspond with. But planetary bodies may also exhibit "short-wavelength" features such as mountains and volcanic arcs that divert rivers away from the long-wavelength path. In such cases, downstream flow directions of rivers are not particularly well correlated with the large-scale landscape topography. Benjamin A. Black et al. compared drainage patterns on Earth, Mars and Titan with the short- and long-wavelength features by developing and analyzing two new topographic metrics. Their technique reveals that drainage systems match better with long-wavelength topography on Titan and Mars than on Earth, where the short-wavelength features dominate. The authors conclude that this is because Earth exhibits active plate tectonics, which continuously generate new short-wavelength features. The utility of these analyses for understanding the geologic history of planetary bodies is highlighted in an accompanying Perspective by Devon Burr.