Art Meets Cartography: The 15,000-Year History of a River in Oregon Rendered in Data. source
The shades of white show changes in elevation, between 0 to 50 feet. This brings out the changes made by the river channel in the last 12,000 to 15,000 years, in the time since the landscape was basically swept clean by the Missoula floods. post 
The beautiful map seen here of the Willamette River Historical Stream Channels in Oregon by cartographer Dan Coe also shows the history of a river, however Coe relied on more recent aerial radar technology called lidar.
It is possible to strip buildings and vegetation from the images, so that only the ground is shown. In the Willamette River poster, the shades of white and blue show elevations. The purest white color is the baseline, (the zero point, at the lowest point near Independence on the upper part of the image). The darkest blue is 50 feet (or higher) than the baseline.