Why does the Red River of the North do like it do? Because the country it flows through is so flat. How flat?
… the river flows very slowly across a pancake-flat landscape. Imagine raising an eight-foot-long sheet of plywood just enough to slip a single sheet of paper under the raised end. The resulting minuscule tilt of the board represents the average slope of the Red River’s bed.
This, by Kirk Johnson, is good writing: visual and accessible, it creatively uses everyday props to dimension a distinctive state of affairs and how it affects events (and redeems the stale pancake cliché).