  We study the kinematics of the giant H II regions NGC 5455 and NGC 5471
located in the galaxy M101, using integral field observations that include the
Hbeta and [O III] 5007 emission lines, obtained with the Keck Cosmic Web
Imager. We analyse the line profiles using both single and multiple Gaussian
curves, gathering evidence for the presence of several expanding shells and
moving filaments. The line decomposition shows that a broad (sigma = 30-50
km/s) underlying component is ubiquitous, extending across hundreds of pc,
while a large fraction of the narrow components have subsonic line widths. The
supersonic turbulence inferred from the global line profiles is consistent with
the velocity dispersion of the individual narrow components, i.e. the global
profiles likely arise from the combined contribution of discrete gas clouds. We
confirm the presence of very extended (400 - 1200 km/s) low-intensity line
components in three bright star-forming cores in NGC 5471, possibly
representing kinematic signatures of supernova remnants. For one of these, the
known supernova remnant host NGC 5471 B, we find a significantly reduced [O
III]/Hbeta line ratio relative to the surrounding photoionized gas, due to the
presence of a radiative shock at low metallicity. We explore the systematic
width discrepancy between H I and [O III] lines, present in both global and
individual spaxel spectra. We argue that the resolution of this long-standing
problem lies in the physics of the line-emitting gas rather than in the
smearing effects induced by the different thermal widths.
