Visualize an expressway. Its entrance ramps point in the direction of the trafﬁc ﬂow, so that trafﬁc merges smoothly with it. That is also how tributaries normally enter rivers—angling downstream, their current merging smoothly with the river’s. If the ﬂow of trafﬁc were reversed, cars would need to almost reverse direction to enter the expressway from an on-ramp. That is analogous to what happens in a barbed drainage, a peculiar drainage pattern that results from stream piracy. Given the right conditions of soil and slope, a river erodes upstream, its bed cutting ever more deeply into the slope it descends. Eventually, it cuts across the height of land and into the headwaters of the opposite slope. It then siphons off those headwaters, because its channel lies below. Thus westward-oriented tributaries now enter a channel that ﬂows eastward, or vice versa. Represented schematically, the “captured” tributaries look like the barbs on a porcupine quill.