Although crevasse splays are often considered important contributors to floodplain-basin filling, evidence from modern and ancient fluvial deposits indicates that they may not play a significant role in all river systems. In order to better understand controls on the abundance and scale of crevasse splays produced in fluvial systems, we used aerial photography to observe crevasse-splay deposits along three rivers and conducted a targeted numerical-modeling study to explore factors which promote widespread crevasse-splay deposition on fluvial floodplains. Results demonstrate that splay size does not always scale directly with channel size or discharge and that hydrodynamic factors—primarily sediment size and floodplain-drainage conditions—play a significant role in determining whether a fluvial system is likely to produce large, floodplain-filling splays. This work suggests that widespread crevasse splays demand both a large supply of relatively coarse suspended sediment and a steep water-surface slope away from the channel across the floodplain. Without both these conditions, the role of crevasse splays in flood-basin filling is limited.