  We present 21cm HI line observations of the blue compact dwarf galaxy
NGC1705. Previous optical observations show a strong outflow powered by an
ongoing starburst dominating the HII morphology and kinematics. In contrast,
most of the HI lies in a rotating disk. An extraplanar HI spur accounts for ~
8% of the total HI mass, and is possibly associated with the HII outflow. The
inferred mass loss rate out of the galaxy's core is significant ~ 0.2 - 2
M_sun/yr, but does not dominate the HI dynamics. Mass model fits to the
rotation curve show that the dark matter (DM) halo is dominant at nearly all
radii and has a central density \rho_0 \approx 0.1 M_sun/pc^3: ten times higher
than typically found in dwarf irregular galaxies, but similar to the only other
mass-modelled blue compact dwarf, NGC2915. This large difference strongly
indicates that there is little evolution between dwarf irregular and blue
compact dwarf types. Instead, dominant DM halos may regulate the morphology of
dwarf galaxies by setting the critical surface density for disk star formation.
Neither our data nor catalogue searches reveal any likely external trigger to
the starburst in NGC1705.
