Lock-release experiments are performed focusing upon the evolution of near-pure glycerol flowing into fresh water. If the lock-height is sufficiently tall, the current is found to propagate for many lock lengths close to the speed predicted for energy-conserving gravity currents. The current then slows to a near-halt as the current head runs out of fluid. The initial rapid advance is not due to a lubrication layer forming below the current. Rather it is due to the front rolling out over the bottom, pushing the ambient fluid up and over the head. Entrainment via the turbulent shear flow over the head is found to be negligible. The front slows only as the head height reduces to that comparable to the boundary layer underneath the head. A conceptual model is developed that captures the transition from an inertially-driven current to its sudden near-arrest by viscous forces.