This set of images from the Mars Global Surveyor shows a comparison of the gully site as it appeared on Dec. 22, 2001 (left), with a mosaic of two images acquired after the change occurred (the two images are from Aug. 26, 2005, and Sept. 25, 2005). The photographs have revealed bright new deposits seen in two gullies on Mars that suggest water carried sediment through them sometime during the 3.5 years interval between the images.

Source: http://marsprogram.jpl.nasa.gov/mgs/newsroom/20061206a.html