When considering the historical path of a river, it’s easy to imagine a torrential flood that causes a stream to overflow its banks, or a drought that brings a body of water to a trickle. The reality of a river’s history is vastly more complex, as the artery of water gradually changes directions over thousands of years, shifting its boundaries imperceptibly inch by inch.
Geologists and cartographers have grappled with helpful ways to visually depict a river’s flow over time. In 1941, the Mississippi River Commission appointed Harold Fisk to undertake a groundbreaking effort to map the entire Lower Mississippi Valley. Three years later he produced a stunning series of 15 maps that combine over 20 different river paths obtained through historical charts and aerial photography.
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. From The Oregonian:
Lidar data is collected by low-, slow-flying aircraft with equipment that shoots millions of laser points to the ground. When the data is studied, an amazingly accurate model of the ground can be mapped.
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.
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.