After exceptionally heavy rainfalls and associated land movements in July 1985, sediment inflow into Lake Tutira, Hawke's Bay, New Zealand, caused a five‐fold reduction in the depth at which 5% of the surface photosynthetically active radiation remained. Light level reduction was associated with increased nephelometric turbidity caused by the suspended sediment. The decreased light levels, as monitored by measurement of the vertical attenuation coefficient of light in the lake, persisted for about three months and were accompanied by a marked reduction in the lower depth limit of aquatic macrophytes: from 6.7 to 1.8 m forElodea canadensis, and from 4.6 to 1.8 m forHydrilla verticillata. As water quality returned to its pre‐storm condition, these species regrew very rapidly in the denuded areas from surviving rhizomes.