  In addition to a smooth component of WIMP dark matter in galaxies, there may
be streams of material; the effects of WIMP streams on direct detection
experiments is examined in this paper. The contribution to the count rate due
to the stream cuts off at some characteristic energy. Near this cutoff energy,
the stream contribution to the annual modulation of recoils in the detector is
comparable to that of the thermalized halo, even if the stream represents only
a small portion (~5% or less) of the local halo density. Consequently the total
modulation may be quite different than would be expected for the standard halo
model alone: it may not be cosine-like and can peak at a different date than
expected. The effects of speed, direction, density, and velocity dispersion of
a stream on the modulation are examined. We describe how the observation of a
modulation can be used to determine these stream parameters. Alternatively, the
presence of a dropoff in the recoil spectrum can be used to determine the WIMP
mass if the stream speed is known. The annual modulation of the cutoff energy
together with the annual modulation of the overall signal provide a "smoking
gun" for WIMP detection.
