  Erosion shapes our landscape and occurs when a sufficient shear stress is
exerted by a fluid on a sedimented layer. What controls erosion at a
microscopic level remains debated, especially near the threshold forcing where
it stops. Here we study experimentally the collective dynamics of the moving
particles, using a set-up where the system spontaneously evolves toward the
erosion onset. We find that the spatial organization of the erosion flux is
heterogeneous in space, and occurs along channels of local flux $\sigma$ whose
distribution displays scaling near threshold and follows $P(\sigma)\sim
J/\sigma$, where $J$ is the mean erosion flux. Channels are strongly correlated
in the direction of forcing but not in the transverse direction. We show that
these results quantitatively agree with a model where the dynamics is governed
by the competition of disorder (which channels mobile particles) and particle
interactions (which reduces channeling). These observations support that for
laminar flows, erosion is a dynamical phase transition which shares similarity
with the plastic depinning transition occurring in dirty superconductors. The
methodology we introduce here could be applied to probe these systems as well.
