  Transition from laminar to turbulent flow drastically changes the mixing,
transport, and drag properties of fluids, yet when and how turbulence emerges
is elusive even for simple flow within pipes and rectangular channels. Unlike
the onset of temporal disorder, which is identified as the universal route to
chaos in confined flows, characterization of the onset of spatio-temporal
disorder has been an outstanding challenge because turbulent domains
irregularly decay or spread as they propagate downstream. Here, through
extensive experimental investigation of channel flow, we identify a distinctive
transition with critical behavior. Turbulent domains continuously injected from
an inlet ultimately decayed, or in contrast, spread depending on flow rates.
Near a transition point, critical behavior was observed. We investigate both
spatial and temporal dynamics of turbulent clusters, measuring four critical
exponents, a universal scaling function and a scaling relation, all in
agreement with the (2+1)-dimensional directed percolation universality class.
