Impacts of Fluctuating Water Levels in the St. Lawrence

Fluctuations in water levels and flows in the St. Lawrence River, whether natural or the result of human activities, are a concern among stakeholders involved in managing the water and its uses and for scientists who are working to protect the fluvial ecosystem.

In 1999, the International Joint Commission (IJC) called for a study on the criteria applicable to the regulation of water levels in Lake Ontario and the St. Lawrence River. Scientists, including specialists from Environment Canada, were mandated to predict the response of certain environmental indicators to various regulation scenarios, among other things. An environmental synthesis report titled Water Availability Issues for the St. Lawrence Riverwas published at the close of this five-year study. Some of its findings are highlighted below.

Plant Productivity and Diversity

St. Lawrence wetlands cover vast communities of riparian and aquatic plants that have characteristics related to those of their environment. The water level is a determining element in the location, biomass and annual production of riparian plants. Indeed, riparian plants are influenced by the average water level of the season in progress as well as by the water-level conditions of the previous year.

Riparian plants endure hydrological conditions that can range from periodic flooding to constant immersion. But they also tolerate changes in physical conditions like wind, luminosity, turbidity, temperature and waves. Additional anthropogenic pressures -- modification of the river bed and bank, ice jam mitigation, regulation of the water level, introduction of species and inputs of contaminants, for example -- are also brought to bear. These conditions, which change over time, influence colonization in some areas and affect the succession and diversity of shoreline vegetation.

Submerged plants are a fundamental element of aquatic habitats and food webs. The effects of hydrology on wetlands reverberate higher up the food chain; low-water conditions promote habitats suitable for some aquatic birds and mammals, while high water expands habitats favoured by invertebrates and fish.

Studies show that there exists a positive correlation between mean water levels and average depth and current speed, and a negative correlation with turbidity, transparency and average water temperature (Hudon et al., 2003b). For example, under low water conditions, the productivity of submerged aquatic vegetation should be greater because of lower average depth and current speed and the upsurge in water temperature and bottom light intensity.

Species Composition and Distribution of Plant Communities

Between 1998 and 2002, scientists noticed a pattern whereby average water levels (1998, 2000 and 2002) alternated with low (1999 and 2001) water conditions. In between seasons of average water levels and low levels, shoreline plant communities shifted to areas nearer the water, providing a prime example of the reaction of St. Lawrence wetlands to interannual variations in water levels.

Some 207 species of emergent and submerged plants were identified in 630 quadrats between Cornwall and Trois-Rivières over four years of sampling. An analysis of the grouping identified plant communities corresponding to a progressive depth and elevation gradient. There are groups of plants characteristic of drier, upland environments, groups from wetter environments (deeper in the water) and groups that are somewhere in between these two extremes.

• Under low-water conditions, plant communities from drier stages are more strongly represented and a bare area appears to act as a transition zone from emergent to submerged environments.
• The drying out of wetlands during low-water periods can facilitate the invasion of exotic or invasive species along the St. Lawrence. Graminoids such as Phalaris arundinacea and Phragmites australis as well as certain facultative annuals have invaded dried-out marshes. For example, Alisma plantago-aquatica, a perennial emergent that propagates vegetatively, as well as young shoots ofLythrum salicaria (< 20 cm), show a marked tendency to colonize the lower reaches of the shore during periods of low water (Figure B).
• The following year, when water levels increase to average values, more-aquatic plant communities and species again increase in size, to the detriment of other communities.
  

Biomass and Productivity of Wetland Plants

Interannual variations in water levels also influence the vertical distribution of the plant biomass. When water levels are low, the shore presents a discontinuous plant cover where the biomass of obligatory aquatic plant species drops in favour of the biomass of the facultative plant species found in these environments. These conditions, which vary over time, affect colonization in some areas and influence the succession and diversity of plants growing along the shore.

The proportions of different types of plants (emergent plants, submerged plants, algae), expressed as biomass, change depending on the hydrological conditions. On dried-out mudflats, for example, submerged plants are replaced by upland annual species. Under low-water conditions, the biomass of upland and emergent plants is greater at higher tiers (i.e. higher up on shore), while the biomass of submerged plants rises in shallow water (0.5 m deep). Denuded areas experience successive immersion and drought, conditions that hinder the establishment of purely aquatic or terrestrial species, and they are characterized by a low plant biomass.

It should be noted that the low flows of 1999 and 2001 correspond to the flows anticipated for the St. Lawrence River and the Great Lakes under climate change scenarios ― meaning a chronic flow reduction on the order of 40% and a one-metre drop in the average water level (Hudon et al. 2003a). Water levels in the St. Lawrence–Great Lakes Basin should be regulated in consideration of a changing climate and the attendant impacts on wetlands. Fluctuating water levels in the St. Lawrence–Great Lakes Basin promote the growth of more diversified wetlands than would occur in a hydrographic system with no fluctuations. Thus, the stabilization of water levels would have harmful repercussions on wetland diversity by reducing the amplitude of seasonal variations.

Generally speaking, the overall biodiversity of an ecosystem is greater when it experiences moderate disruptions on a periodic basis. If too intense or too frequent, however, these disturbances will reduce biodiversity by eliminating the sensitive species in favour of the resistant ones. Scientists believe that a hydrological cycle in which seasonal and interannual variations mimic, as much as possible, natural conditions, has the effect of minimizing impacts on the productivity and diversity of aquatic plants.

Fish and Fish Habitats

Hydrological variations can affect fish at different spatial and temporal scales. In the short term (intra-annual variations), flow and level fluctuations can modify the local distribution, seasonal range and migratory pattern of species, with direct effects on commercial and sport fish catches.

Restricted access to spawning grounds and sheltered areas in rivers can modify fish breeding behaviour. In the long term, variations in the hydrological regime can influence the population dynamics of fish by acting on their reproductive and recruitment processes, changing the relative abundance of the affected species.

A tagging study conducted between 1999 and 2001 in the Quebec City area provided a prime example of potamodromy (migration in fresh water) in more than 20 species of St. Lawrence freshwater fish (de Lafontaine et al., 2002). These species, including Yellow Perch (Perca flavescens), Walleye (Sander vitreus), Sauger (Sander canadensis), Channel Catfish (Ictalurus punctatus), White Sucker (Catostomus commersoni), Longnose Sucker (Catostomus catostomus) and White Perch (Morone americana), travel seasonally over a distance of some 200 km along the fluvial section between the Sorel islands, the western edge of Lake Saint-Pierre and the eastern tip of île d’Orléans.

This seasonal pattern -- migrating upstream in fall and downstream in spring -- has been observed in many St. Lawrence species. It does not appear to be related to imminent reproductive requirements, however, because most St. Lawrence freshwater species breed in the spring. Rather, as documented in other northern rivers, seasonal variations in habitat accessibility appear to be at issue.

Based on records of daily fish catches at the experimental fishery in Saint-Nicolas between 1975 and 2002, scientists observed that the time and duration of the seasonal migration of 20 species of fish varied greatly from one year to the next and that this can be attributed to variations in the hydrological regime. The downstream migration in spring was delayed during years of heavy flooding. By contrast, the migration upstream in fall was later in years with higher water levels in late summer/early fall.

Fish habitat quality can suffer as a result of low water levels by reducing the surface area of spawning grounds and limiting access to them; egg hatching rates can also be lower. Developing eggs require specific flow conditions and temperatures, and the presence of submerged vegetation.

Floodplains function as springtime spawning grounds for many species of fish, including Northern Pike, which seek out the dense vegetation found in high marshes. Under extreme low water levels, however, marshes, with their less-than-optimal vegetation, serve as spawning grounds. Low breeding success means less abundant populations of Northern Pike.

The integrity of the flood cycle must be preserved to ensure the maintenance of a well diversified and resilient fish community. Specific flood control and/or level or flow regulation measures to benefit a particular species are not recommended: such actions, repeated over many years, would necessarily affect the other components of the ecosystem. In the worst-case scenario, the floodplain would be stabilized, an intolerable situation for those fish species that have adapted to this variable and temporary environment. Where there are no floodplains, there are few fish. Given its present status, a natural approach should be taken toward the river's hydrological regime to avoid transforming the fluvial ecosystem into a canal.

Herpetofauna

It is possible to observe 20 species of amphibians in the St. Lawrence Valley, along with 17 species of reptiles. Although these numbers appear low next to the 4900 species of amphibians and 8950 reptiles that have been surveyed worldwide (Pough et al., 2000), Quebec has the highest diversity of herpetofauna in Eastern Canada (Desroches and Rodrigue, 2004).

The life cycle of most anurans (frogs and toads) can be divided into three distinct stages: egg, larval and adult. Generally speaking, the eggs are laid in water with the larvae emerging and metamorphosing into adults a few weeks later. Larval reproduction and growth therefore constitute life stages that are particularly sensitive to hydrological and climatic factors for anurans. Egg production in most species extends from April to mid-June, with larvae developing from mid-June to the end of August.

During this period, the eggs and larvae can be dried out by a sudden drop in the water level. Aquatic anurans like Bullfrogs or Green Frogs are more sensitive to fluctuating water levels, especially given that at our latitudes, their larval stages last for two to three years.

The specific diversity and abundance of amphibian larvae increases as a function of their immersion in water. We should not, however, conclude that a lengthy flood period is necessarily favourable to anuran richness or abundance. In fact, a number of predators are associated with permanent or semi-permanent water bodies, including insects of the Anax genus (dragonflies), salamanders of the genus Ambystoma and fish.

Although St. Lawrence aquatic turtles breed in upland riparian environments, water-level fluctuations can still have repercussions on them. Females seek denuded sandy or clay soils close to the water to lay their eggs. Such proximity to the water increases the flood risk to their nests and the ensuing death of the embryos.

Fluctuations in water levels modify the area of wetlands that are habitats for anurans and turtles (Barko et al., 1999). The area taken up by marshes dominated by emergent plants is directly related to water levels, so lower-than-average levels would shrink the size of all habitats favoured by amphibians and reptiles.

Hibernating anurans and Testudines (tortoises and turtles) may be forced to confront the indirect effects of the prevailing hydrological conditions. Indeed, high mortality rates are observed in winter when unusually cold temperatures collide with low water levels. Any lowering of the water level in winter increases the risk of killing anurans and aquatic turtles, which hibernate on water bottoms.

Regulation scenarios for the St. Lawrence were assessed based on the availability of habitats vital to the maintenance of anuran populations. While the influence of level fluctuations and climate change on St. Lawrence herpetofauna were considered, it must be borne in mind that they are not the only environmental factors placing pressure on these animals. Other stressors include habitat fragmentation, chemical pollution, infectious disease, introduced species, disturbance, illegal harvesting and intentional destruction at the hands of humans.

Learn More

Talbot, A. (dir.). 2006. Water Availability Issues for the St. Lawrence River - An Environmental Synthesis. Environment Canada, Montreal. 182 pages.

Literature

Barko, J.W., J.D. Madsen, and T.D. Wright. 1999. Ecological Effects of Water Level Reductions in the Great Lakes Basin. For the International Joint  Commission.

de Lafontaine, Y., F. Marchand, D. Labonté, and M. Lagacé. 2002. The Hydrological Regime and Fish Distribution and Abundance in the St. Lawrence River: Are Experimental Trap Data a Valid Indicator? Scientific report submitted to the International Joint Commission. Environment Canada – Quebec Region, Environmental Conservation, St. Lawrence Centre. 33 pages.

Desroches, J.-F. and D. Rodrigue. 2004. Amphibiens et reptiles du Québec et des Maritimes. Éditions Michel Quintin. 288 pages.

Hudon, C., J.-P. Amyot, and C. Plante. 2003a. Répartition verticale des communautés de plantes aquatiques en fonction des variations de niveau du Saint-Laurent. Scientific report submitted to the International Joint Commission. Environment Canada – Quebec Region, Environmental Conservation, St. Lawrence Centre.

Hudon, C., P. Gagnon, C. Vis, J.-P. Amyot, and D. Rioux. 2003b. Models for Submerged Vegetation and Related Environmental Changes Induced by Discharge (Water Level) Variations in the St. Lawrence River (Québec, Canada). Report presented to the International Joint Commission as part of the international study on Lake Ontario and the St. Lawrence River by the Environmental Technical Working Group. November 2003.

Pough, H.F., A.M. Robin, J.E. Cadle, M.L. Crump, A.H. Savitzky, and K.D. Wells (eds.). 2000. Herpetology. 2nd Edition. Prentice Hall Inc., Englewood Cliffs, New Jersey. 612 pages.

Robichaud, A. and R. Drolet. 1998. Fluctuating Water Levels in the St. Lawrence River. Fact sheet in the "State of the St. Lawrence River" series. Federal-provincial SOE team, composed of representatives of Environment Canada, Fisheries and Oceans Canada, and the Ministère de l’Environnement et de la Faune du Québec, Sainte-Foy.