This report also suggests that some channels may have been obscured by a sedimentary cover as has been found on Earth. The hidden channels on Earth were only revealed by ground-penetarting radar. Missions with GPR such as Mars Express 2003 and Mars Reconnaissance Orbiter 2005 may also reveal such hidden channels on Mars:

ERASURE OF FIRST-ORDER TRIBUTARIES VIA CLIMATE CHANGE: LESSONS FOR MARS FROM
EARTH. Ted A. Maxwell, J.A. Grant, B.A. Campbell, R. Irwin III, M. Bourke and A. Johnston, Center for Earth
and Planetary Studies, National Air & Space Museum, Smithsonian Inst., Washington, D.C. 20560
tmaxwell@nasm.si.edu
Lunar and Planetary Science XXXIV (2003) 2049.pdf
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/2049.pdf

Bob Clark