A key prediction of LCDM is that galactic halos are filled with sub-halos orbiting within them, whereas warm dark matter models would have few. Studies of individual Milky Way star streams find evidence of a few sub-halos, but the sub-halo population properties are unclear. Current stream studies have focused on the high surface density (~34 mag/square arcsec!) core of the stream.  The arrival of a number of massively multiplexed spectrographs opens up the opportunity to acquire spectra of an unbiased sample of stars along and around stellar streams that will provide the kinematic data required to compare to predictions. The velocity dispersions in streams should, on the average, rise from ~5 km/sec near the stream origin/minimum, to about 40 km/sec at 20-30 degrees, the details depending on the sub-halo numbers.  Measurement of the velocity dispersion-angular distance relation will test for the numbers of halos in the ~10^7-8 M_sun range, and, provide data for complementary studies.