Waves, rivers, and tides play a leading role in shaping delta morphology. Recent studies have enabled predictions of their relative influence for deltas globally, but methods and associated uncertainties have remained poorly described. Here we aim to address that gap and assess the quality of delta morphology predictions compared to observations for 31 deltas globally. We expand on seminal works that quantified the Galloway ternary diagram from the balance between river, wave, and tidal sediment fluxes. Our data includes uncertainties for delta shoreline protrusion angles set by wave influence (14.1°±12° predicted vs. 20.8°±16.1° observed), channel widening, set by tidal influence (53.5±170.8 predicted vs. 6.5±11.5 observed), and number of distributary channels, set by river influence (55.9±127.5 predicted vs. 21.4±43.0 observed). Within the ternary diagram for delta morphology, we find an average error of 8% (±11%, 1 standard deviation), linked to uncertainties in wave and tide sediment fluxes. Relative uncertainties are greatest for mixed-process deltas (e.g., Sinu, error of 49%) and tend to decrease for end-member morphologies where either one of wave, tide, or river sediment fluxes dominates (e.g., Fly, error of 0.2%). Large sources of prediction uncertainties are (1) delta morphology data, e.g., delta slopes that modulate tidal fluxes, (2) data on river sediment flux distribution between individual delta river mouths, and (3) theoretical basis behind fluvial and tidal dominance. Future work could help address these three sources and improve predictions of delta morphology.