The Tharsis region of Mars is covered in vast lava flows, many with channels. Some channels, however, resemble features that may have been formed by water.
In this image, we see a smooth, flat-bottomed channel within the roughly irregular edges of a possible lava flow. This long, winding channel runs for 115 kilometers (70 miles) from its source (shown in ESP_045091_2045), maintaining a nearly consistent width. There is also a streamlined island within the channel, which is 1.25 kilometers (about 3/4 mile) long.
One possibility is that a lava flow formed, and later groundwater was released, preferentially flowing through and further eroding the pre-existing lava… [More at link]