We investigated the seasonality of life history stages in the freshwater red alga Thorea okadae in the Kikuchi River (Kumamoto, Japan), a broad (24–72 m wide), stony stream with marked annual variation in current velocity and discharge (10–350 m3 s−1). Gametophytes were observed from early autumn (September) to late spring (April-May), and gametangial formation began in December and peaked in March. Fertilization occurred primarily in March and April, and despite a sparse distribution of gametophytes in the river, a high percentage of female gametophytes were fertilized (82%). Chantransia stages (sporophytes) persisted year-round, although their estimated percent cover on rocks in the warm season (August-September) was greater than in the cold season (January-May). The appearance of gametophytes paralleled a decrease in day length, and consequently, a decrease in total irradiance and water temperature. We suggest that these environmental conditions were proximate factors affecting phenological changes in T. okadae and that variation in river discharge (unstable and high vs stable and low) may ultimately have driven these life history patterns in this macroalga. In situ growth measurements indicated that no elongation and sometimes a length reduction occurred in gametophytes during the growing period, while rapid growth of gametophytes (65-cm increase in 43 days) was observed at the same time. These results and in situ observations suggest that the distal portions of gametophytes were frequently broken off and that many gametophytes were entirely swept away even in the growing season.