Impacts of the Petitcodiac Causeway

Causeway Construction in 1967

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These 1967 aerial photos show the construction of the Petitcodiac River causeway, approximately 500 m in length, and the causeway gate structure, on solid land in Riverview. Sediment deposits flank the downriver side, where Moncton later located its landfill operation. A combination of rock and sediment deposits occurs upstream.
The 700-foot (215 m) opening in the causeway creates one-foot (0.3 m) tidal difference, suggesting that a slightly larger opening is required to create «full tidal flow or exchange» conditions (i.e. zero tidal difference). An opening approximately 250 m wide has been considered. The 1967 photo provides a useful model to predict the effects of restoring « full tidal flow » to the Petitcodiac River.

Biodiversity and Ecosystem

The causeway consists of a physical barrier that obstructs fish passage and migration on approximately half of the 3,000 km2 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-1990’s.

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 2,000 to 3,000. 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 have been eliminated due to the causeway construction include Atlantic salmon (except for stocking), American shad, Atlantic tomcod, and striped bass. Species whose population have been greatly reduced include 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 4.7 km of 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 have reduced the river’s width from 1.6 km before 1968 to 120 m in 1998

Upstream sedimentation is on the one hand caused by saltwater inflow through the fishway and, when the tide is 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 enter the reservoir, there is virtually no way they can return downstream and as a result they accumulate continuously on the bed. If nothing is done about the present situation, it is estimated that the headpond will become a freshwater marsh within a few decades.

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