Planetary Exporation Program

Superposition

New geologic features typically form on top of older ones. Where two features meet, the one on top is generally the newer feature. So, when you see an area where two features (like a crater and a river) meet, you can tell which is newer if you can figure out which one is on top.

At the center of the above image there appears to be a river running north/south with two impact craters in it. The elevation cross section supports the visual interpretation. The riverbed is lower than surrounding terrain and the craters are lower still.
If these craters formed before the river, they should have been destroyed when the river formed over it. It is hard to see how such a large river couldn't fill in these two large holes. However, if the river formed first and then the craters we'd be left with what we see above.