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We investigate 3D size-segregation in a granular avalanche. A thin bi-disperse avalanche is brought to rest within a confined geometry, forming a stationary deposit. The front of the stationary deposit propagates upslope as a shock in the flow thickness. The shock freezes the segregation state of the disperse avalanche at a given point into the deposit. 3D magnetic resonance images of the deposit have been acquired to obtain particle distribution characteristics. Significant gradients in lateral and downstream concentration profiles are observed. Initially, faster evolution of the segregation state occurs at the walls, with the core remaining closer to the inlet conditions. Ultimately, at larger flow distances, a core rich in large particles develops, with layers of predominantly small particles at the side-walls and base. Evidence from both graded and inversely graded inlet conditions suggests that the avalanches converge to the same stable segregation state for distances larger than ∼ 1300 large particle diameters. Crucially, the pure-small-particle layers that form at the base of the flows, whilst initially varying in thickness, converge towards the same constant thickness by the chute’s end wall. Although the non-constant thickness of the pure-small-particle layer at smaller flow distances is not always reported in prior work on chute avalanches, there is a good qualitative agreement on the evolution of the segregation state down the chute with prior studies. [ABSTRACT FROM AUTHOR]