It depends whom you ask, of course.

Vic Baker would say that we have no reason to seek anything other than water, because that is sufficient to produce the features we see, and it is the simplest fluid we know of, and is proven to work on Earth.

I might place the boot on the other foot and suggest that CO2 outbursts were more reasonable on Mars, given the prevailing temperatures.

I am in the middle of an attempt to explore the situation by analysing the nature of the overbank deposits.

For those who are not practicing geologists, an overbank deposit is what you get when a river in flood spills out over its banks and sets off across the floodplains. Because its velocity slows compared to that in the main channel, it often deposits material in distinctive banks and streams.

In principle, one can use the nature of these to tell you about the type of fluid. Lavas and mudflows leave thick pasty lobes. Dilute agueous floods leave striated banks, and Gas-supported flows tend to spread out laterally and may reform secondary flows some distance from the primary flood channel.

That's the theory. In practice, on Mars after 2 or 3 Ga of aeolian erosion and meteorite bombardment, the evidence is obscure. I had hoped for an answer from MGS, but the answer is not so easily won. I am continuing my search...

Recently, another idea was tossed into the fray, suggesting that dust itself might flow like liquid under Mars surface conditions. Original Message Here

Whilst I find it difficult to believe that whole canyons could be carved by such fine dusts before the winds whipped them away, there may be some local value to this concept, especially with the dark dust streaks.

In summary, it ain't easy, but I, and others, are working on aspects of the problem. If you have some useful comments or observations to add, I would appreciate them much more than these guessing games.