  Rings and gaps are being observed in an increasing number of disks around
young stellar objects. We illustrate the formation of such radial structures
through idealized, 2D (axisymmetric) resistive MHD simulations of coupled
disk-wind systems threaded by a relatively weak poloidal magnetic field
(plasma-$\beta \sim 10^3$). We find two distinct modes of accretion depending
on the resistivity and field strength. A small resistivity or high field
strength promotes the development of rapidly infalling `avalanche accretion
streams' in a vertically extended disk envelope that dominates the dynamics of
the system, especially the mass accretion. The streams are suppressed in
simulations with larger resistivities or lower field strengths, where most of
the accretion instead occurs through a laminar disk. In these simulations, the
disk accretion is driven mainly by a slow wind that is typically accelerated by
the pressure gradient from a predominantly toroidal magnetic field. Both
wind-dominated and stream-dominated modes of accretion create prominent
features in the surface density distribution of the disk, including rings and
gaps, with a strong spatial variation of the magnetic flux relative to the
mass. Regions with low mass-to-flux ratios accrete quickly, leading to the
development of gaps, whereas regions with higher mass-to-flux ratios tend to
accrete more slowly, allowing matter to accumulate and form dense rings. In
some cases, avalanche accretion streams are observed to produce dense rings
directly through continuous feeding. We discuss the implications of ring and
gap formation driven by winds and streams on grain growth and planet formation.
