We contrasted diets of three turtles (Elseya albagula, Myuchelys latisternum, Emydura krefftii) from free-flowing or impounded rivers in southeastern Queensland, Australia, to evaluate the effects of flow regulation. The turtle species encompassed the herbivorous, carnivorous, and omnivorous feeding guilds, respectively. The study design simultaneously considered ontogenetic dietary shifts and seasonal effects on prey availability. Relative to the river samples, diets for three turtles in impoundments were substantially reduced in prey abundance, species richness, and dietary breadth. Turtles with narrower dietary preferences in free-flowing rivers were more affected by dams than was the dietary generalist. For the most omnivorous turtle, qualitative differences were evident in diets from impoundments and rivers; yet considering its greater dietary breadth, quantitative effects on diet were minor. Diets of turtles in impoundments included fewer subaquatic plants and wind fallen fruits than did the diets of turtles in rivers. For the largely carnivorous turtle, fewer aquatic invertebrates were ingested in impoundments than in riverine habitats, and scavenging behavior was noted mainly in impoundments. Multidimensional scaling of the site characteristics identified dams or weirs with back-ups exceeding 20 km as being relatively similar in impacts on prey diversity. Canonical correspondence analysis identified factors of habitat alteration and turtle size as major determinants of underlying variance in the diets. The results suggest that turtle food webs are altered by river regulation. A general finding that turtle diets in impoundments were depleted of aquatic plants or macro-invertebrates pertains to other turtles of conservation importance.