  How do flocks, herds and swarms proceed through disordered environments? This
question is not only crucial to animal groups in the wild, but also to
virtually all applications of collective robotics, and active materials
composed of synthetic motile units. In stark contrast, appart from very rare
exceptions, our physical understanding of flocking has been hitherto limited to
homogeneous media. Here we explain how collective motion survives to
geometrical disorder. To do so, we combine experiments on motile colloids
cruising through random microfabricated obstacles, and analytical theory. We
explain how disorder and bending elasticity compete to channel the flow of
polar flocks along sparse river networks akin those found beyond plastic
depinning in driven condensed matter. Further increasing disorder, we
demonstrate that collective motion is suppressed in the form of a first-order
phase transition generic to all polar active materials.
