I had complained earlier this week that geological evidence that Martian surface rocks have been cold for almost their entire history is hard to reconcile with Spirit and Opportunity findings of surface rocks with clear water alteration. The distinction seems to be between surface temperature, where the sun maybe can heat things, and deeper rocks. In any case, Science Now discusses the geology another very visible watery feature of Mars -- the huge arroyos, especially in the Valles Marineris system. These appear to be transient, catastrophic flood formations. A new hypothesis says the water was there, possibly trapped in sulfate salts, and released all at once maybe by vulcanism.
For fans, here's a whole poster session on whether, or how much, water mars has seen.
In the mean time, check out this perspective image of a (dry) canyon wall in Mars, from the USGS.