  Deep observations of faint surface brightness stellar tidal streams in
external galaxies with LSST are addressed in this White Paper contribution. We
propose using the Wide--Fast--Deep survey that contains several nearby galaxies
(at distances where the stars themselves are not resolved, i.e., beyond 20
Mpc). In the context of hierarchical galaxy formation, it is necessary to
understand the prevalence and properties of tidal substructure around external
galaxies based on integrated (i.e., unresolved) diffuse light. This requires
collecting observations on much larger samples of galaxies than the Milky Way
and M31. We will compare the observed structures to the predictions of
cosmological models of galactic halo formation that inform us about the number
and properties of streams around Milky Way-like galaxies. The insight gained
from these comparisons will allow us to infer the properties of stream
progenitors (masses, dynamics, metallicities, stellar populations). The changes
in the host galaxies caused by the interactions with the dissolving companion
galaxies will be another focus of our studies. We conclude by discussing
synergies with WFIRST and Euclid, and also provide concrete suggestions for how
the effects of scattered light could be minimized in LSST images to optimize
the search for low surface brightness features, such as faint unresolved
stellar tidal streams.
