  We report measurements in a 2-dimensional, gravity-driven, collisional,
granular flow of the normal force delivered to the wall and of particle
velocity at several points in the flow. The wall force and the flow velocity
are negatively correlated. This correlation falls off only slowly with distance
transverse to the flow, but dies away on the scale of a few particle diameters
upstream or downstream. The data support a picture of short-lived chains of
frequently colliding particles that extend transverse to the flow direction,
making transient load-bearing bridges that cause bulk fluctuations in the flow
velocity. The time-dependence of these spatial correlation functions indicate
that while the force-bearing structures are local in space, their influence
extends far upstream in the flow, albeit with a time-lag. This leads to
correlated velocity fluctuations, whose spatial range increases as the jamming
threshold is approached.
