A lot of people have complained that there are factual errors regarding this graphic. Here's some background information about this particular print. This dates back to 1854 when, I imagine, the only way to measure the length of river systems was to start at one end and travel to the other, measuring all the way. The transcontinental railroad was still 15 years in the future. Much of that part of the country was still unexplored and unsettled.

So yeah, there's a couple errors. Let's see anyone here do better (without using Google Earth).

In addition, if you're wondering what the numbers correspond to, the legend can be found here.