Water motion has wide-ranging effects on scleractinians, particularly during thermal bleaching when increased flow may aid in resistance to high temperature. Increased flow removes photosynthetically-derived oxygen radicals, causing greater efficiency of PSII and conferring resistance to bleaching. However, enhanced flow can have both positive and negative effects on coral photophysiology (light-adapted yield, QY). Modest increases in flow benefit QY, while larger increases can have detrimental effects, suggesting that there is a threshold effect in QY along a flow continuum. To reconcile these opposing trends, we first compared the growth of juvenile Porites spp. among flow microenvironments in a lagoon to determine whether flow affects growth with a comparable threshold effect, reasoning that this might reflect a fitness consequence of similar influence on QY. Second, with juvenile Porites spp., we used a flume to test the hypothesis that an interaction between flow and temperature could explain the contrasting effects of flow on QY. Growth of Porites spp. in lagoon microenvironments responded with the anticipated threshold effect. In the flume, QY at 28.3?C displayed a threshold effect with a maximum at 23 cm/s, but this effect was absent at 31.1 ?C where QY increased linearly with flow to 43 cm/s. Finally, using juvenile Porites spp. we tested the duration of these effects, reasoning that these patterns may be short lived. For this analysis, corals were exposed to contrasting flow speeds under high temperature and light in the flume for 6 hrs, measuring QY every 30 min; QY was consistently ≈ 47% higher in high versus low flow. Together, our results indicate the potential for threshold effects of flow on growth of Porites spp. in the field, and demonstrate a threshold effect of flow on photophysiology at ambient temperature in the flume; the stimulatory effects of flow persist for at least 6 h. Thus, our results may provide a means to reconcile previous inconsistencies in the effect of flow on corals.