The Sedimentary History of Lake Saint-Pierre

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The Sedimentary History of Lake Saint-Pierre

Lake Saint-Pierre is the downstreammost of the freshwater basins of the St. Lawrence. It is a major component of the fluvial ecosystem, with sedimentary processes typical of deltaic zones.

With the exception of the 11-metre-deep ship channel that bisects it, Lake Saint-Pierre is characterized by its shallowness, its mean depth being no more than three metres. (See Bathymetry and Current Velocity in the Fluvial Lakes). This characteristic influences the lake’s hydrodynamics, limiting the formation of permanent sediment-accumulation zones in the upstream sector. Lake Saint-Pierre is more of a sediment transition zone, where sediments are deposited temporarily during summer and evacuated in winter under the ice cover or during the subsequent spring freshet.

Grain Size of Surface Sediments

The lake bed is composed primarily of zones of fine sediment, sand and gravel atop the marine clays of the Champlain Sea; this is characteristic of the St. Lawrence Lowlands. Surface-sediment sampling campaigns conducted in Lake Saint-Pierre in 2000 and 2004 by Environment Canada yielded more than 100 samples. Grain-size analyses revealed that most were made up of fine to medium-grained sand.

Thickness of Surface Sediment

The northern portion of the lake is relatively free of accumulated sediment. Sediment thickness varies from 10 to 25 cm of fine sand deposited directly over postglacial clays. The area downstream of the Sorel delta has a sediment-accumulation rate of roughly 1.8 cm per year. Sediments can reach up to 245-cm thick in the elongation of Îles de la Girodeau, Île de la Traverse, Île Plate and Île de Grace, thus forming a zone of permanent sediment accumulation — that is to say, a zone in which sediment inputs exceed losses.

Zones of Permanent Sediment Accumulation

Sediment-accumulation zones are generally located outside the main channel, where currents are slower than 0.3 m/s. Thus, the hydrodynamic characteristics of the lake influence the rate of sedimentation at some sites. In addition to the area downstream of the Sorel delta, we observe two other sedimentation zones in the southern portion of the lake. One area is downstream of the delta of the Saint-François River, where sediments are composed largely of fine, compacted sand and where the sediment accumulation rate has varied from 2 to 3 cm a year for more than a half century. The other area is situated at the mouth of the Nicolet River, where we find mainly silty sediments and a rate of sedimentation that has not yet been determined precisely.

Carte : Zone sédimentation permanente au lac Saint-Pierre

Sampling of Deepwater Sediments

Four stations in the upstream portion of Lake Saint-Pierre were depth-sounded to sample ancient sediments. The mean water depth at the four sampling stations is about 2.5 m and the sedimentation processes in these sectors are typical of deltaic zones, where the deposition of particles is favoured by a rapid drop in current speed.

20-kg hammer corer	Barge with central trap



Splitting sediment cores

Sediment core taken from sampling station No. 2 on Lake Saint-Pierre

The sediment core taken from sampling station No. 2 shows more than 2.5 m of accumulated sediment, made up of alternating layers of unreworked sand and silt over an area of 1 m (central portion of core). This confirms the presence of a permanent sedimentation zone. Dating of the core provides indicators of the sedimentary dynamics at this site for over 100 years. We are thus able to determine the relationship between the natural and anthropogenic events that have altered the sedimentary dynamics of the St. Lawrence, including Lake Saint-Pierre.

We observe silty layers that have been deposited during the summer, corresponding to the start of construction of overflow weirs in the Berthier–Sorel islands. The rate of sedimentation at the time was 2.9 cm/year and it remained stable until the early 1960s, the period corresponding to the regulation of water levels and the opening of the Seaway, as well as icebreaking in winter. These impacts on the hydrodynamic characteristics of the system cause the resuspension of silt deposited in summer.


The Montreal flood of 1886	Opening of the Seaway and regulation of water levels (1959–1965)

Construction of overflow weirs between 1928 and 1930

Prior to 1925, observations suggest that sediment deposition in Lake Saint-Pierre was influenced by episodes of heavy runoff. During the spring freshet, ice jams would form in the region of Trois-Rivières, raising the water level all the way up to Montreal. The current was strong enough to transport fine and medium-grained sand, leaving only the medium to coarse sand downstream of the Sorel delta.

In the area of the Berthier–Sorel islands, five channels (Chenal aux Castors, Petit chenal de l’Île Dupas, Chenal aux Ours, Le Grand Chenal and Chenal des Barques) were closed by weirs beginning in 1928 to raise the water level in the Port of Montreal and increase flows in the ship channel during low-water periods.

Chemical Quality

The results of geochemical analyses show that the chemical quality of surface sediments has improved since 1986. Indeed, we observe a 94% reduction in concentrations of organic contaminants (PCBs) and a drop of more than 50% in concentrations of inorganic contaminants (Cu, Hg, Cd and Cr) in the northern portion of the lake. These concentrations are below the sediment quality criteria for the protection of aquatic life.

Concentrations of mercury

Sediment core taken from sampling station No. 2 on Lake Saint-Pierre

A profile of the history of the chemical quality of core No. 2 reveals significant increases in mercury concentrations in the early 1950s, corresponding to the operation of industrial plants upstream of Lake Saint-Pierre. These concentrations dropped considerably following the closure of these facilities in the 1990s.

Concentrations of PBDE₄₇

Sediment core taken from sampling station No. 2 on Lake Saint-Pierre

Analytic work conducted in 2004 shows an average concentration of 10.8 ng/g of PBDE_total (24 congeners) in surface sediment in the Sorel delta of Lake Saint-Pierre; the highest concentrations can reach 92 ng/g (30.3 ng/g excluding PBDE₂₀₉). Of the congeners analysed, PBDE₄₇, one of the most toxic, presents average concentrations of around 1.2 ng/g. Concentrations were highest in the area downstream of this archipelago and in the northern portion of Lake Saint-Pierre.

An analysis of core sample No. 2 indicates that PDBE₄₇was present in this part of Lake Saint-Pierre in the mid-1970s. Concentrations climbed steadily upward until the 1990s, a period corresponding to the increased commercial use of this product, when they peaked at 4 ng/g. Since then, levels have varied from 2.0 to 2.5 ng/g, according to the observations of Environment Canada scientists. It should be noted that no criteria applicable to PBDEs have been adopted to date.

**Concentrations of PBDE ₄₇ measured in** sediment
in different rivers and lakes

The average levels of PBDE₄₇ in Lake Saint-Pierre are the highest of many of the world’s aquatic ecosystems.

PCB concentrations

Overall PCB concentrations have fallen by approximately 90–95% since 1986. It is highly likely that this reduction is related to the Montreal wastewater cleanup program and the generalized decline in PCBs in the St. Lawrence system over the last 20 years.

Looking Ahead

Work is under way to

take a snapshot of how the sedimentary dynamics of Lake Saint-Pierre have evolved over the past 100 years;
determine the impacts of numerous anthropogenic events on the sedimentary dynamics of the fluvial system;
provide a historic overview of changes in contaminants, vegetation and climate.

Literature

Burton, J. 1991. Le lac Saint-Pierre, document d’intégration. Zone d’intervention prioritaire 11. Environment Canada – Quebec Region, Environmental Conservation. 98 pages.

Loiselle, C., G.R. Fortin, S. Lorrain, and M. Pelletier. 1997. Dynamics and Contamination of St. Lawrence River Sediment. St. Lawrence UPDATE series. Environment Canada – Quebec Region, Environmental Conservation, St. Lawrence Centre. Montreal.

Pelletier, M. 2005. Toxic Contamination of Sediments – Lake Saint-Pierre: Last Stop before the Estuary. Fact sheet in the “Monitoring the State of the St. Lawrence” series. Environment Canada – Quebec Region and Ministère du Développement durable, de l’Environnement et des Parcs du Québec.