  We present a novel method for determining the total matter surface density of
the Galactic disk by analysing the kinematics of a dynamically cold stellar
stream that passes through or close to the Galactic plane. The method relies on
the fact that the vertical component of energy for such stream stars is
approximately constant, such that their vertical positions and vertical
velocities are interrelated via the matter density of the Galactic disk. By
testing our method on mock data stellar streams, with realistic phase-space
dispersions and Gaia uncertainties, we demonstrate that it is applicable to
small streams out to a distance of a few kilo-parsec, and that the surface
density of the disk can be determined to a precision of 6 %. This method is
complementary to other mass measurements. In particular, it does not rely on
any equilibrium assumption for stars in the Galactic disk, and also makes it
possible to measure the surface density to good precision at large distances
from the Sun. Such measurements would inform us of the matter composition of
the Galactic disk and its spatial variation, place stronger constraints on dark
disk sub-structure, and even diagnose possible non-equilibrium effects that
bias other types of dynamical mass measurements.
