Construction of the Petitcodiac River Causeway in 1968 converted 21 km of macrotidal (tidal amplitude > 8 m) brackish-water estuary into a shallow impoundment. The ecological value of the freshwater impoundment was low, when judged by criteria of temporal stability, uniqueness, conservation of native species, and level of disturbance. By interfering with migrations, the causeway dam eliminated or greatly reduced the populations of most diadromous fishes, including a large population of the now-endangered Inner Bay of Fundy stock of Atlantic salmon (Salmo salar). Loss of the host fish for the parasitic glochidial stage resulted in extirpation of the dwarf wedgemussel (Alasmidonta heterodon) from Canada. Benthic and planktonic communities of the impoundment were depauperate, and typical of a disturbed ecosystem. Most species were of marine/estuarine origin, or belong to freshwater groups with rapid dispersal. Illegally introduced warm-water fishes flourished in the impoundment, presumably replacing the native species.