One current theory that has apparently been given some serious thought is that these are definately water channels.
The strongest evidence is from the Mars rover which sent back pictures of the surface.
When there is a flood event the water pushes rocks across the ground surface and the flow of water grades them by size.
If you look at those pictures you will see that for the most part the rocks are sitting on top of the sediment layer and are pretty similar in size and shape, the direction of flow can be determined by the way water has scoured around the rocks and indeed the scouring void behind the rocks is all in the same direction thus proving a cohesive flow.
This phenomenon is well known on earth, particularly in Iceland where sub-ice sheet volcanic fractures cause massive sudden flood events.
The current model has it that there were vast underground reservoirs of liquid water trapped by a deep layer of permafrost, this water could not freeze solid becuase of immense pressure.
Some event such as a deep impacting asteroid fractured the permafrost layer and the water was forced out in a relativelyu short period of time, the flood covered a large part of the suface of Mars.
There are collapsed sink hole structures resembling those on earth where the ground simply fell in.
The water evaporated away in the thin Martian atmospere and weak gravitational field.
That’s the theory anyway but I wonder about the orbit of Mars. If it lost such a large mass of material into space through evaporation then would not the orbit have changed, either in speed or radius ?