  We find new integrable boundary conditions, depending on a free parameter
$g$, for the O(N) nonlinear $\sigma$ model, which are of nondiagonal type, that
is, particles can change their ``flavor'' through scattering off the boundary.
These boundary conditions are derived from a microscopic boundary lagrangian,
which is used to establish their integrability, and exhibit integrable flows
between diagonal boundary conditions investigated earlier. We solve the
boundary Yang-Baxter equation, connect these solutions to the boundary
conditions, and examine the corresponding integrable flows.
