  We use a coarse-grained model of superconducting vortices driven through a
random pinning potential to study the nonlinear current-voltage (IV)
characteristics of flux flow in type II superconductors with pinning. In
experiments, the IV relation measures flux flow down a flux density gradient.
The work presented here treats this key feature explicitly. As the vortex
repulsion weakens, the vortex pile maintains a globally steeper slope,
corresponding to a larger critical current, for the same pinning potential. In
addition, the magnitude of the peak in the differential resistance falls as the
resistance peak shifts to higher currents. The model also exhibits so-called
"IV fingerprints", and crossover to Ohmic (linear) behavior at high currents.
Thus, many of the varieties of plastic behavior observed experimentally for
soft flux line systems in the ``peak regime'' are reproduced in numerical
simulations of the zero temperature model. The nonlinear transport behaviors
are related to the self-organized, large scale morphologies of the vortex river
flow down the slope of the vortex pile. These morphologies include isolated
filamentary channels, braided rivers, and flooded rivers. We propose that these
self-organized morphologies of flux flow down a flux gradient govern the
various plastic flow behaviors, including nonlinear IV characteristics,
observed in type II superconductors.
