In or about 1883, a northward avulsion of the meandering lower Saskatchewan River at the Cumberland Marshes initiated a broad belt of alluvial sedimentation that has continued to evolve to the present, and now covers some 500 km 2 The avulsion belt comprises a complex alluvial terrain dominated by individual and coalesced splays and associated wetlands connected by networks of active and abandoned channels of various sizes. The Windy Lake splay, a predominantly fine-grained feature typical of large parts of the avulsion belt, was examined to investigate the relationships between splay evolution and facies development. The splay, now largely inactive, mostly formed over a 35-year period, depositing nearly 2 m of sediment over a 7.2 km 2 area. The splay surface is characterized by stable, well-defined anastomosed distributary channels that separate interchannel wetland basins. Heights of channel levees diminish downstream, and the levees merge with channel-mouth bars whose progradation forms the locus of lakeward channel extension and the base for subsequent subaerial levee deposition. The anastomosed channel pattern was created when the mouth bars of separate distributary channels merged during progradation. Vibracore and soil-probe profiles show the splay to be dominated by upward-coarsening lacustrine-prodeltaic mouth-bar facies sequences capped by levees and organic-rich interchannel wetland deposits. Channels, now containing sand only sporadically in narrowed or slightly migrated reaches, will eventually fill with fine sediment when abandonment is complete. The Windy Lake splay is typical of stage III splays (Smith et al. 1989), whose deposits and associated lacustrine fills form at least half of the avulsion belt. Thin, widespread, predominantly upward-coarsening sequences capped by either nondepositional horizons or true overbank fines may be characteristic of many avulsive suites in the ancient alluvial record.