"For me, the most compelling evidence for recent water on mars are the pictures of flows on crater walls."

As far as I can tell nothing compel me to think it's water rather than something like Nick's CO flows.

"What if, like on Earth, water is contained in aquifers. It may not be evenly distributed. Granted this water is frozen.
Is it possible that a meteor strike might "un-earth" the ice and raise the temperature long enough to melt the ice and create the flows like the ones seen in the picture?"

My point was assuming that there was an atmosphere- I wasn't arguing whether or not Mars could or could not have an atmosphere [that's a separate argument].
Assuming Mars' atmosphere had enough density to allow lakes and rivers similar to Earth would mean Mars would also have weather similar to Earth [lots of clouds]. Also since the air is denser, Mars' wind would have more power to lift dust/sand off the ground and into the air. Therefore if there was any dust/sand "available on the surface" it would probably spend a large amount of time in the atmosphere.
In other words if the Earth was mostly land area and that land had no vegetation, wouldn't you expect the dust/sand storms as fair common event? Now add in Mars' 1/3 gravity- pigs could fly [and bricks too:].
So on Mars in order to have lakes and rivers (which weren't choked with dirt thereby not be lakes and rivers) you would need no or little wind or no or little dust/sand. Currently, Mars has lots of wind and dust/sand.