Which 1930s project mapped the Mississippi's every historical channel in colour?

Harold Fisk's survey for the Army Corps of Engineers, published in 1944 - a set of maps showing every former course of the lower Mississippi in a different colour, layered over one another like marbled paper. They are among the most reproduced technical drawings ever made, mostly by people who have no idea what they are for.

The purpose was flood control. A meandering river continually cuts new channels and abandons old ones, leaving oxbow lakes and scars readable in aerial photographs and soil borings; Fisk reconstructed that history to argue about what the river would do next and where levees should go. The maps are a record of a river that has never stayed put and does not intend to.

The current course is now held in place by engineering. The Mississippi has been trying to jump into the Atchafalaya, a shorter and steeper route to the Gulf, for decades - it would have done so around 1975 - and is prevented by the Old River Control Structure, which nearly failed in the 1973 flood. Keeping the river where it is keeps Baton Rouge and New Orleans on a working waterway, and the maps show exactly how unnatural that is.