But the remnants of about 300 craters have been found. Click on this diagram of a river drainage system for an illustration of how small scale tributaries combine to form a large scale river system. Looking at the figures in Chapters 7-10. The images below show the same region of Mars taken at two different times. And the surface is too cold for liquid water.
Any water is frozen in the polar caps or in the soil.
BUT there is evidence of erosion in the past - suggesting that at one point the atmosphere was denser, the surface warmer and water flowed in rivers, canyons. There is also evidence of occasional catastrophic flooding. It is probable that the sulfuric acid in the clouds rains out and causes some chemical erosion of the rocks.