by Ty Fischer, Riparian Health Restoration Intern
Reaching up to hundreds of kilometers wide and hundreds of meters deep, it is perhaps not surprising that the Laurentian Great Lakes share many geological similarities with saltwater oceans. They have high winds, large waves, and even complex current systems that in many ways make them dynamically closer to being inland seas than typical freshwater lakes (Rao, Schwab, 2007). As a result of these strong hydrological processes, their coastlines are similarly dynamic and in a state of constant change. This factor has important implications for anyone with a vested interest in the state of the Great Lakes’ shorelines. This blog will provide an introduction to the physical processes in these vast bodies of water, describe how they manifest physically on their coastlines, and propose how climate change is expected to alter these areas in the future.
The Great Lakes – Superior, Huron, Ontario, Erie, and Michigan – experience a wide range of hydrological processes that are unique among freshwater lakes in North America. Covering a total area of 244,000km2 and containing over 18% of the world’s supply of freshwater (Waples et al., 2008), they are so large that they are influenced by the rotation of the Earth and form strong currents as a result (Rao, Schwab, 2007).
The massive surface area of the Great Lakes means they have much more fetch than typical lakes, leading to intense winds. In fact, average wind stress on the Great Lakes is even higher than that in open ocean, when all factors are accounted for (Boyce et al., 1989). Such high winds create vigorous surface waves, seiches (standing waves with longer periods of water-level oscillations), and storm surges (Gronewold et al., 2013), all of which are uncommon on Canada’s freshwater lakes. Lastly, the Great Lakes are subject to daily, seasonal, and decadal water-level fluctuations, arising as a result of a variety of natural and human-induced factors (Neff, Nicholas, 2005).
The typical water level fluctuations, which can range from 31cm across years and up to 2m across decades (Buckman, 2019), directly influence the shoreline vegetation in these areas. The fluctuations increase the area that shoreline vegetation can establish in, increase the diversity of types of vegetation and species, and vastly alter vegetation distributions due to the ever-changing substrate moisture levels.
For instance, periods of elevated water levels prevent the establishment of woody vegetation and other terrestrial species close to the water but can kill dominant species such as cattails. Conversely, low water levels allow mud flat annuals, meadows, and emergent vegetation to regenerate from buried seeds. In such a way, the plant communities bordering the Great Lakes differ in distribution and composition from most other freshwater lakes in Canada as they generally display a higher marsh area and greater diversity of vegetation types and plant species (Keddy, Reznicek, 1986).
Another unique attribute of the Great Lakes’ coastlines is that the high winds, intense waves, storm surges, and seiches lead to increased instances of flooding and erosion, especially when water levels are high (Boyce et al., 1989). Among other impacts, these events further contribute to the unique structure of these shorelines through eroding, suspending, and redistributing sediment.
These events are observed along coastal bluffs, the dominant shoreline type in the Great Lakes. Coastal bluffs are created when upland areas face heavy sediment losses at the base due to wave erosion (Mackey, 2012), and sediment that is then resuspended in the water through these processes can redistribute by current and wave action. This eventually accumulates to form beaches (Mickelson et al., 2004). Combined with the natural water level variability, the uncommonly strong winds, waves, seiches, and storms make these shorelines both highly variable and highly dynamic. This can lead to challenges when approaching projects relating to property rights (like urban development) and public interests (like shoreline restoration) (Buckman et al., 2019).
An extra layer of complexity arises due to global climate change. Firstly, the predicted increased precipitation, storm frequency and severity during the winter and spring, and drought conditions in summer and fall are likely to affect short-term, seasonal, and interannual water variability. These changes can alter native species distributions and can even make some areas more vulnerable to invasion by foreign species such as Phragmites.
Climate change will also bring higher than average winter and spring precipitation which will increase nutrient and sediment runoff into the Great Lakes. This is likely to further alter the composition of plant communities on shorelines and may result in more algal and cyanobacterial blooms. Third, warmer air and water temperatures are likely to cause ecotonal shifts in aquatic species distributions (Mackey, 2012). In the winters, these changes have already begun to reduce ice cover thus leaving the exposed beaches, shorelines, and bluffs more susceptible to flooding and erosion (Bartolai et al., 2015). These are just a few of the projected impacts, but all in all, climate change is poised to contribute to a series of changes that will have measurable impacts on the form and function of the Great Lakes and their surrounding coastlines.
Between the water-level changes operating on multiple timescales, disturbances from winds, waves, seiches, and storms, and long-term variability from climatic changes, the Great Lakes are truly unique freshwater lakes. They display a wide variety of hydrogeomorphic characteristics and physical processes (Mackey, 2012), and they are highly dynamic ecosystems that are in a state of constant change (Wensink, Tiegs, 2016). Practically, this means that restoration projects on their coastlines must account for an inherent level of variability. Restorations must strive to build resilient ecosystems which are not only effective at re-establishing native habitat, mitigating erosion, and reducing runoff, but which can surmount this state of constant change and provide lasting benefits long into the future. The specifics of how this can be achieved will be detailed in part two of this blog series.
References
Bartolai, A. M., He, L., Hurst, A. E., Mortsch, L., Paehlke, R., & Scavia, D. (2015). Climate change as a driver of change in the Great Lakes St. Lawrence River Basin. Journal of Great Lakes Research, 41, 45–58. https://doi.org/10.1016/j.jglr.2014.11.012
Boyce, F. M., Donelan, M. A., Hamblin, P. F., Murthy, C. R., & Simons, T. J. (1989). Thermal structure and circulation in the Great Lakes. Atmosphere-Ocean, 27(4), 607–642. https://doi.org/10.1080/07055900.1989.9649358
Buckman, S., Arquero de Alarcon, M., & Maigret, J. (2019). Tracing shoreline flooding: Using visualization approaches to inform resilience planning for Small Great Lakes Communities. Applied Geography, 113, 102097. https://doi.org/10.1016/j.apgeog.2019.102097
Gronewold, A. D., Fortin, V., Lofgren, B., Clites, A., Stow, C. A., & Quinn, F. (2013). Coasts, water levels, and climate change: A great lakes perspective. Climatic Change, 120(4), 697–711. https://doi.org/10.1007/s10584-013-0840-2
Keddy, P. A., & Reznicek, A. A. (1986). Great Lakes Vegetation Dynamics: The role of fluctuating water levels and buried seeds. Journal of Great Lakes Research, 12(1), 25–36. https://doi.org/10.1016/s0380-1330(86)71697-3
Mackey, S. D., 2012: Great Lakes Nearshore and Coastal Systems. In: U.S. National Climate Assessment Midwest Technical Input Report. J. Winkler, J. Andresen, J. Hatfield, D. Bidwell, and D. Brown, coordinators. Available from the Great Lakes Integrated Sciences and Assessments (GLISA) Center, http://glisa.msu.edu/docs/NCA/MTIT_Coastal.pdf.
Neff, B. P., & Nicholas, J. R. (2005). Uncertainty in the Great Lakes Water Balance. Scientific Investigations Report. https://doi.org/10.3133/sir20045100
Rao, Y. R., & Schwab, D. J. (2007). Transport and mixing between the coastal and offshore waters in the Great Lakes: A Review. Journal of Great Lakes Research, 33(1), 202–218. https://doi.org/10.3394/0380-1330(2007)33[202:tambtc]2.0.co;2
Waples, J. T., Eadie, B., Klump, J. V., Squires, M., Cotner, J., & McKinley, G. (B. Hales, W.-J. Cai, G. Mitchell, C. L. Sabine, & O. Schofield, Eds.), North American Continental Margins: A Synthesis and Planning Workshop 73–80 (2008). Washington, DC; U.S. Carbon Cycle Science Program.
This blog post is part of a Climate Change toolkit, generously funded by The Catherine and Maxwell Meighen Foundation. Access the full toolkit here.