Many Mars scientists support the view that many of the channels seen on Mars formed subglacially which wouldn't show the same geomorphic features as precipitation induced water carved channels.
Also, in the upcoming Conference on Early Mars will be presented reports suggesting valley networks in Valles Marineris at least are consistent with formation by atmospheric precipitation:

PALEOLAKE AND DENSE VALLEY NETWORKS ORIGINATED BY PRECIPITATIONS WITHIN MELAS
CHASMA, VALLES MARINERIS.
"Conclusion: Atmospheric precipitations have
occurred on Melas Chasma at least during several
thousand years forming dense hydrographic networks
and a paleolake. Melas Chasma which formed during
the Hesperian period demonstrates that wet and warm
conditions allowing an efficient water cycle existed
after 3.5 Gy on Mars."
Second Conference on Early Mars (2004) 8010.pdf
http://www.lpi.usra.edu/meetings/earlymars2004/pdf/8010.pdf

NEW OBSERVATIONS OF VALLEY NETWORKS USING THEMIS (MARS ODYSSEY) AND HRSC
DATA (MARS EXPRESS).
Second Conference on Early Mars (2004) 8019.pdf
"Conclusion: Valleys observed in the Valles Marineris
region display dendritic shape and morphometric characteristics
of terrestrial river systems. They can be
dated to the Late Hesperian epoch from chronological
relationships. Surface runoff in the Late Hesperian
epoch could correspond either to a progressive transi-
tional climate after the warmer Noachian epoch [13] or
to episodic warmer periods like those which could be
related to the increase in atmospheric water vapor due
to the outburst of outflow channels [14]. HRSC data
will permit to considerably improved the current
knowledge of valley networks on Mars."
http://www.lpi.usra.edu/meetings/earlymars2004/pdf/8019.pdf

Bob Clark