Lava and glacial erosion would have produced very different channels. If so, the channels also are evidence that the climate on Mars has changed dramatically. Of course, liquid water cannot exist on Mars today. If any buried water reached the surface, the planet's low atmospheric pressure would boil it to a vapor. Once in the atmosphere, the water vapor would circle the poles and freeze out.
Water channels suggest a time in Mars history when the atmosphere was much thicker.
That would mean the climate once was warmer and wetter. Counting craters in the dry river channels suggest they are several hundred million years old. There is evidence the water which carved the channels is locked in polar ice caps buried under the two large carbon dioxide frost caps. That water, plus some frozen in the Martian soil, might be enough to form an ocean in the low-lying northern hemisphere. It might be several miles deep if melted.