  We study the dynamics of Bose-Einstein condensates in an optical lattice and
harmonic trap. The condensates are set in motion by displacing the trap and
initially follow simple semiclassical paths, shaped by the lowest energy band.
Above a critical displacement, the condensate undergoes Bragg reflection. For
high atom densities, the first Bragg reflection generates a train of solitons
and vortices, which destabilize the condensate and trigger explosive expansion.
At lower densities, soliton and vortex formation requires multiple Bragg
reflections, and damps the center-of-mass motion.
