  The outflow driven by the low-mass class 0 protostar L1157 is the prototype
of the so-called chemically active outflows. The bright bowshock B1 in the
southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD)
shock models, for which dynamical and chemical processes are strongly
interdependent. We present the first results of the unbiased spectral survey of
the L1157-B1 bowshock, obtained in the framework of the key program "Chemical
Herschel Surveys of Star Forming Regions" (CHESS). The main aim is to trace the
warm and chemically enriched gas and to infer the excitation conditions in the
shock region. The CO 5-4 and H2O lines have been detected at high-spectral
resolution in the unbiased spectral survey of the HIFI-Band 1b spectral window
(555-636 GHz), presented by Codella et al. in this volume. Complementary
ground-based observations in the submm window help establish the origin of the
emission detected in the main-beam of HIFI, and the physical conditions in the
shock.}{Both lines exhibit broad wings, which extend to velocities much higher
than reported up to now. We find that the molecular emission arises from two
regions with distinct physical conditions: an extended, warm (100K), dense (3e5
cm-3) component at low-velocity, which dominates the water line flux in Band~1;
a secondary component in a small region of B1 (a few arcsec) associated with
high-velocity, hot (> 400 K) gas of moderate density ((1.0-3.0)e4 cm-3), which
appears to dominate the flux of the water line at 179mu observed with PACS. The
water abundance is enhanced by two orders of magnitude between the low- and the
high-velocity component, from 8e-7 up to 8e-5. The properties of the
high-velocity component agree well with the predictions of steady-state C-shock
models.
