  The existence of partially ionized, diffuse gas and dust clouds at kiloparsec
scale distances above the central planes of edge-on, galaxy discs was an
unexpected discovery about 20 yrs ago. Subsequent observations showed that this
EDIG (extended or extraplanar diffuse interstellar gas) has rotation velocities
approximately 10-20% lower than those in the central plane, and have been hard
to account for. Here we present results of hydrodynamic models, with radiative
cooling and heating from star formation. We find that in models with star
formation generated stochastically across the disc an extraplanar gas layer is
generated as long as the star formation is sufficiently strong. However, this
gas rotates at nearly the same speed as the mid-plane gas. We then studied a
range of models with imposed spiral or bar waves in the disc. EDIG layers were
also generated in these models, but primarily over the wave regions, not over
the entire disc. Because of this partial coverage, the EDIG clouds move
radially, as well as vertically, with the result that observed kinematic
anomalies are reproduced. The implication is that the kinematic anomalies are
the result of three-dimensional motions when the cylindrical symmetry of the
disc is broken. Thus, the kinematic anomalies are the result of bars or strong
waves, and more face-on galaxies with such waves should have an asymmetric EDIG
component. The models also indicate that the EDIG can contain a significant
fraction of cool gas, and that some star formation can be triggered at
considerable heights above the disc midplane. We expect all of these effects to
be more prominent in young, forming discs, to play a role in rapidly smoothing
disc asymmetries, and in working to self-regulate disc structure.
