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Impacts of the Petitcodiac River causeway construction
Biodiversity and Ecosystem
The causeway consists of a physical barrier
that obstructs fish passage and migration to approximately half
of the 3000 km2 river system. The causeway was built with very
little regard for estuarine 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 2000 to 3000. 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, 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 (1000 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. (1967 aerial photos)
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 headpond
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. |
