QUESTION:
Could the channels have been formed from liquids other than water?
Lava, for example?

ANSWER from Dr. Robert Anderson on December 11, 1996:
This is an excellent question.  On Earth, it is very easy to seperate 
channels that resulted from volcanic activity versus channels which 
resulted from fluvial (water) processes.  On Mars we can identify both
types of channels.  Whether the channels were formed by water is
another question.  A lot of research has been done to examine other
mechanisms for channel development.  Some of these include liquid
carbon-dioxide, liquid sulfur-dioxide, etc.  But all of these would
require major changes in the atmosphere of Mars.  Fluvial processes
are still the number one candidate for the formation of the channels.