  We present the first complete, velocity-resolved [CII] 158um image of the M51
grand-design spiral galaxy, observed with the upGREAT instrument on SOFIA.
[CII] is an important tracer of various phases of the interstellar medium
(ISM), including ionized gas, neutral atomic, and diffuse molecular regions. We
combine the [CII] data with HI, CO, 24um dust continuum, FUV, and near-infrared
K-band observations to study the evolution of the ISM across M51's spiral arms
in both position-position, and position-velocity space. Our data show strong
velocity gradients in HI, 12CO, and [CII] at the locations of stellar arms
(traced by K--band data) with a clear offset in position-velocity space between
upstream molecular gas (traced by 12CO) and downstream star formation (traced
by [CII]). We compare the observed position--velocity maps across spiral arms
with synthetic observations from numerical simulations of galaxies with both
dynamical and quasi-stationary steady spiral arms that predict both tangential
and radial velocities at the location of spiral arms. We find that our
observations, based on the observed velocity gradients and associated offset
between CO and [CII], are consistent with the presence of shocks in spiral arms
in the inner parts of M51 and in the arm connecting the companion galaxy, M51b,
in the outer parts of M51.
