Impacts on the Ecosystem

Biodiversity and Ecosystem

Owned and operated by the Province of New Brunswick, the Petitcodiac causeway, built in 1968, dramatically altered natural ecosystem functions in the 2000 km2 Petitcodiac River ecosystem, creating an artificial 21 km headpond (called a “lake” by opponents of the restoration) and obstructing fish passage to over two thirds (1,340 km2) of the river system.

The causeway was built with very little regard for ecology and conservation concerns for one of the most important macrotidal estuaries in Canada. Important aquatic biota and estuarine habitats were deeply affected by the Petitcodiac River causeway construction. In the years following the construction, anadromous fish populations dramatically declined in the river system. Entire stocks and native species of the Petitcodiac River became extinct from the system between the mid-1980′s and the mid-1990s.

Fish Migration

Fish migrations were immediately affected by the causeway project due to construction activity and sediment accumulation. Migrating anadromous fish like the Atlantic salmon require freshwater flow (attraction flow) in order to stimulate movement upstream. The causeway obstructed this flow and created a physical barrier for migrating fish. Vertical slot and surface-port fishways were built intending to create this attraction flow of water and to permit upstream fish passage. Extreme sedimentation dissipating the attraction flow and the very high tides of the estuary were the major factors for the inefficiency of the fishway.

Before the causeway construction, estimated annual salmon runs ranged between 3,000 and 5,000. From 1969 to 1972, fish passage investigations indicated 140, 345, 895 and 468 adult salmon entering the river in each respective year. Shad runs in the Petitcodiac were estimated to be in excess of 50,000 to 75,000 fish yearly prior to 1968. A fishway count at the causeway in 1972 totalled only 19 shad. By 1979, federal fisheries scientists were recommending complete removal of the causeway gates as “the best means of assuring fish passage at the causeway.”

Species that were eliminated due to the causeway construction include Atlantic salmon (except for stocking), American shad, Atlantic tomcod, and striped bass. Species whose populations were greatly reduced included sea-run brook trout and rainbow smelt.

Lost Species: Dwarf Wedgemussel

The Petitcodiac River drainage was the only recorded location in Canada for the Dwarf wedgemussel and was one of the only two areas where the species was considered to be common (the other was the Connecticut River system). The Dwarf wedgemussel produces a parasitic larval stage that requires attachment to a fish host for a short period of its life cycle. It is very much likely that eradication of the fish host by the causeway is the cause for the extirpation of the Dwarf wedgemussel in the Petitcodiac River system. The American Shad, which was almost immediately eliminated after the causeway construction, was the most probable candidate for the host of this unique species.

Dam constructions that extirpated fish hosts have been linked to the decline of this Dwarf wedgemussel elsewhere. In the USA, the Dwarf wedgemussel is extirpated from all but 20 of the 70 known locations and is listed as an endangered species. In April 1999, the species was classified as “extirpated in Canada” by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).

Silt Deposits

Massive sedimentation occurred both upstream and downstream during and immediately after the causeway structure construction was finished. An estimated 10 million cubic metres of sediment accumulated in the 4.8 km of the river below the causeway in the first three years (Bray et al. 1982). This massive sedimentation was far greater than what was predicted by the causeway designers. A large mud flat began forming on the downstream side of the causeway even before the construction was complete. This mud flat was estimated to be over 400 ha (1,000 acres) in 1997 (Harvey, 1997). The mud became so dense that approximately 15 % of an old landfill on the Moncton shore, right next to the causeway, now extends out onto the mud flat.

Downstream siltation had raised the riverbed by 3 to 3.7 m by 1979. Stopping the incoming tide at the causeway has caused extreme sedimentation downstream of the causeway. This reduced the river’s width at the causeway by 92% (from 1 km in 1968 to 80 m in 1998) (Naegel and Harvey, 1998). The effects are also evident at Bore Park where silt deposits had reduced the river’s width from 1.6 km before 1968 to 120 m in 1998. By the year 2000, massive silt deposits covered 95 percent of the river near Moncton and extended 35 kilometres downstream to Shepody Bay.

Upstream sedimentation was, on the one hand, caused by saltwater inflow through the fishway and, when the tide was higher than the reservoir level, by leakage through the gates. Up to 3.7 to 4.3 m of silt had accumulated in the head pond in the first ten years of the causeway’s existence, representing 10% of its volume. Once these sediments entered the reservoir, there was virtually no way they could return downstream and as a result they accumulated continuously on the bed. If nothing were done about the situation, it was estimated that the headpond would mostly become a freshwater marsh within a few decades.

Tidal Bore

Once renowned as a natural wonder of the world and considered a top tourism attraction of Atlantic Canada in the 1950s and 60s, the Petitcodiac tidal bore was severely impacted by the construction of the causeway. Reduced to a mere trickle, the subject of ridicule and embarrassment in the community (renamed the “total bore” by cynics) and virtually ignored by the tourism industry, the phenomenon became essentially relegated to the nostalgic realm of urban myths and legends. Between 1969 and 2010, depending on the amplitude of the phenomenon, the wave in the Petitcodiac River would vary from a few cm in height to a maximum of 75 cm (as high as 2 m before the construction of the causeway), and speeds ranging from a few km/hour to 13 km/hour.


Once home to a thriving shipbuilding industry, natural navigational conditions were eliminated near Moncton by 1970, the community thus becoming the first in Canada to lose its inherent right to a navigable waterway.


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