Freshwater Estuaries: Where Wetlands, Rivers, and the Great Lakes Join
Freshwater estuaries are unique Great Lakes coastal wetlands. They occur where rivers and Great Lakes water mix in shallow wetlands located near the mouth of a river. Freshwater estuaries have several important characteristics that are used to define them.
Characteristics of Freshwater Estuaries
A number of freshwater estuary descriptions and definitions exist (Albert et al., 2005; Herdendorf, 1990; Keough, 1986; Mecozzi, 1996; Smith, Ragotzkie, Andren, & Harris, 1988). For the purposes of this report, the recent Glossary of Geology definition is used to describe the attributes characteristic of a freshwater estuary (Neuendorf et al., 2005). The glossary defines a freshwater estuary for the Great Lakes and other large lakes as “the lower reach of a tributary to the lake that has a drowned river mouth, shows a zone of transition from stream water to lake water, and is influenced by changes in lake level as a result of seiches or wind tides” (p. 254). The definition also indicates that freshwater estuaries are commonly separated from the lake by a barrier spit or baymouth bar.
The four primary characteristics of a freshwater estuary outlined in The Glossary of Geology are described in greater detail below.
- Drowned River Mouths
A drowned river mouth occurs when the lower end of a river is submerged or flooded by encroaching water from the Great Lakes. Drowned river mouths are typically the result of geologic factors that have been occurring over thousands of years. At the end of the last Ice Age around 10,000 years ago, massive amounts of ice, which were as much as several hundred feet thick, retreated from much of the Great Lakes Basin (Dott & Attig, 2004). As the ice retreated, the earth’s crust, which had been compressed by the incredible weight of the ice, started to very slowly rebound. The rebounding of the earth’s crust is still occurring today. The crustal rebound has also caused the outlet of the Great Lakes to rebound, sometimes faster than areas downstream. The rise in the outlet level of the Great Lakes has created a subsequent rise in lake water level and the encroachment of Great Lakes water into river valleys, thereby creating drowned river mouth systems (Herdendorf, 1990). In fact, the U.S. Geological Survey (1995) has found that the southwestern portions of Lake Superior have risen approximately 15 to 18 feet over the past 2000 years. They estimate that the lake level rise in those areas is occurring at a rate of one inch per decade. Their study predicts that the rising Lake Superior water levels attributable to crustal rebound will continue to inundate low-lying river mouths and expand wetlands.
- River-Lake Transition Zone
Freshwater estuaries have a zone of transition from stream water to lake water. The mixing of water in this transition zone is influenced by seiche and wind tides (which are described in the next section) and creates unique characteristics. For example, stream water typically has a higher temperature and greater turbidity (i.e., the degree of cloudiness in water caused by suspended solids) than Great Lakes water. The mixing of river and lake water in a freshwater estuary can affect temperature, turbidity, and chemical composition (Herdendorf, 1990). Those affects can, in turn, impact water density; currents; and sediment, nutrient, and contaminant transport, all of which influence important ecological processes.
- Seiche and Wind Tides
The Great Lakes exhibit an interesting natural phenomenon called a “seiche”. A seiche is an oscillation, or periodic back-and-forth movement, that occurs in large waterbodies (Herdendorf, 1990; Neuendorf et al., 2005). One way to visualize a seiche is to imagine a bowl of water that you gently shake. After shaking the bowl, the water moves back-and-forth. The same phenomenon happens in the Great Lakes, only the factors “shaking” the Great Lakes are atmospheric disturbances such as winds or changes in barometric pressure. In waterbodies as large as the Great Lakes, the back-and-forth movement never actually stops and seiche effects can be observed on a daily basis. The intervals, or periods, between seiche peaks on the Great Lakes can range from minutes to more than eight hours (Keillor, 2003). Seiches can cause changes in water surface elevations of a few inches or several feet depending upon atmospheric conditions. Freshwater estuaries experience frequent wet and dry periods, especially near the water margins, due to seiche effects.
A wind tide, or storm surge, is a vertical rise in water level on the leeward, or downwind, side of a waterbody as a result of strong winds. Storm surges on the Great Lakes can produce a change in water level of up to 8 feet under extreme conditions (Keillor, 2003). Given their association with storms and high winds, the effects of a wind tide are often more dramatic than the effects of a seiche. Wind tides can also be a contributing factor to seiche effects.
- Barrier Spits and Baymouth Bars
Freshwater estuaries are commonly separated from the adjacent main body of water by a barrier spit or baymouth bar. Spits and bars are accumulations of sand and gravel in the Great Lakes that can form entirely or partly across the mouth of a river. Many, although not all, freshwater estuaries are partially enclosed by bars or spits.
Historical Perspective
Efforts to define and describe freshwater estuaries have created interesting debate within the scientific community. Estuary science has, for the most part, been historically focused on areas where freshwater from a river mixes with saltwater from the ocean. Marine estuary areas are unique environments that support animals and other organisms adapted to survival in these dynamic systems.
The concept of another estuary-type system occurring at the intersection of a freshwater river and large freshwater lake, such as the Great Lakes, has not always been warmly embraced. The debate surrounding the term “freshwater estuary” is probably best illustrated through a series of papers that were printed in the scientific journal Estuaries in 1990. Dr. Charles Herdendorf from The Ohio State University published an article entitled “Great Lakes Estuaries” (Herdendorf, 1990). The article was a point-by-point description of why the Great Lakes possess estuary-like environments in the lower reaches of some tributary river systems. The article closes by proposing that the term “freshwater” or “Great Lakes estuary” be applied to the drowned portion of streams entering the Great Lakes.
Two scientists from the Marine Sciences Research Center in New York promptly authored a rebuttal article entitled “Great Lakes Estuaries – Phooey” (Schubel & Pritchard, 1990). In the article, they quipped that “Dr. Herdendorf receives our nomination for the prize for the most extensive, most carefully crafted, most eloquent, but least persuasive argument that we have read to gain full estuarine status for Great Lakes tributaries” (p. 508). They further added, “Calling the marginal waterbodies of the Great Lakes estuaries won’t make them estuaries” (p. 508).
A more measured reaction was outlined in a separate response by Dyer (1990), in which he observed, “Estuaries are a microcosm of physical oceanographic and fluid dynamic processes. Freshwater estuaries are a part of that spectrum… Taken constructively, we probably have a lot to learn from [freshwater estuaries]…” (p. 505).
While it is important to recognize the historic scientific debate over freshwater estuaries, it is even more important to recognize that freshwater estuaries have become increasingly recognized and referred to by a wide range of organizations and scientists. For example, an article in the April 1996 issue of the Wisconsin Natural Resources magazine, a publication of the Wisconsin Department of Natural Resources, described freshwater estuaries in the following way (Mecozzi, 1996):
Rivers that slope into the lakes form drowned valleys where lake and river water mix. The Mink River Estuary on Lake Michigan and the Fish Creek, St. Louis, and Bad River estuaries on Lake Superior provide important spawning grounds for fish and valuable staging areas for migrating birds.
The Wisconsin Natural Heritage Inventory Program, which is a Wisconsin Department of Natural Resources’ program that focuses on locating and documenting occurrences of rare species and natural communities, also recently added “Great Lakes Estuary” as a community complex in its natural community classification system (Wisconsin Natural Heritage Program, 2006). In 2005, Albert, Wilcox, Ingram, & Thompson published a hydrogeomorphic classification system for Great Lakes coastal wetlands. In their classification system, “fresh-water estuarine” is identified as an alternative term used by some researchers for “drowned river mouth” systems.
The University of Wisconsin-Extension has been conducting educational programming specifically focusing on freshwater estuaries for over five years, most notably through the Lake Superior Estuary Ed-ventures program offered at the Northern Great Lakes Visitors Center office in Ashland. A video documenting Wisconsin’s Lake Superior freshwater estuaries has been developed through those educational efforts (University of Wisconsin-Extension, 2003), and was highlighted in the 2003 Wisconsin Great Lakes Chronicle published by the Wisconsin Coastal Management Program (Techtmann, 2003). In 2007, The Wisconsin Wetlands Association made Great Lakes freshwater estuaries a topical area for their 12th Annual Wetland Science Forum.
Freshwater estuaries have also been increasingly recognized at the national level. The National Estuarine Research Reserve System (NERRS), a program administered by the National Oceanic and Atmospheric Administration, has included “estuary-type areas of the Great Lakes and their connecting waters” as areas eligible for inclusion in the NERRS program (National Estuarine Research Reserve System Regulations, 2003). As a result, Great Lakes freshwater estuary systems are included within this network of protected areas established for long-term research, education and stewardship. In fact, the Wisconsin Coastal Management Program, University of Wisconsin-Extension, and Wisconsin Department of Natural Resources are leading an effort to designate a freshwater estuary NERRS site on Wisconsin’s Lake Superior south shore (Olson, 2006; Techtmann & Robinson, 2007).
Freshwater estuaries have even been formally defined by national organizations such as the American Geological Institute, which includes a definition for these systems in the Glossary of Geology (Neuendorf, Mehl, & Jackson, 2005).
In summary, freshwater estuaries have become recognized and defined both statewide and nationally, and are an ecological system with important relevance to the Great Lakes region.
References
Albert, D.A., Wilcox, D.A., Ingram, J.W., & Thompson, T.A. (2005). Hydrogeomorphic classification for Great Lakes coastal wetlands. Journal of Great Lakes Research, 31(Supplement 1), 129-146.
Dott, R.H. & Attig, J.W. (2004). Roadside Geology of Wisconsin. Missoula, MT: Mountain Press Publishing Company.
Dyer, K. (1990). The rich diversity of estuaries. Estuaries, 13, 504-505. Retrieved May 2, 2006, from Estuaries and Coasts, Journal of the Estuarine Research Federation, online database.
Herdendorf , C. (1990). Great Lakes estuaries. Estuaries, 13, 493-503. Retrieved May 2, 2006, from Estuaries and Coasts, Journal of the Estuarine Research Federation, online database.
Keillor, P. (2003). Living on the coast: Protecting investments in shore property on the Great Lakes. Madison, WI: University of Wisconsin-Sea Grant Institute; Detroit, MI: U.S. Army Corps of Engineers, Detroit District.
Keough, J.R. (1986). The Mink River - a freshwater estuary. Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, 74, 1-11. Retrieved July 11, 2006, from the University of Wisconsin Digital Collections database.
Mecozzi, M. (1996). Between sky and shore: Wisconsin’s coastal wetlands. Wisconsin Natural Resources Magazine, April 1996 Special Section. Retrieved February 8, 2006, from http://www.wnrmag.com/supps/1996/apr96.htm
National Estuarine Research Reserve System Regulations. 15 C.F.R., § 921.2 (2003).
Neuendorf, K.K.E., Mehl, J.P., & Jackson, J.A. (Eds.). (2005). Glossary of Geology (5th ed.). Alexandria, VA: American Geological Institute.
Olson, T. (2006). Wisconsin Lake Superior National Estuarine Research Reserve. Wisconsin Great Lakes Chronicle 2006, 6-7.
Schubel, J.R. & Pritchard, D.W. (1990). Great Lakes estuaries – phooey. Estuaries (13), 508-509. Retrieved May 2, 2006, from Estuaries and Coasts, Journal of the Estuarine Research Federation, online database.
Smith, P.L., Ragotzkie, R.A., Andren, A.W., & Harris, H.J. (1988). Estuary rehabilitation: The Green Bay story. Oceanus, 31(3), 12-20.
Techtmann, C. (2003). A string of pearls: The estuaries of Chequamegon Bay. Wisconsin Great Lakes Chronicle 2003, 6-7.
Techtmann, C & Robinson, P. (2007). Wisconsin’s Freshwater Estuary Initiative: Developing a Lake Superior Freshwater Estuary Center [Brochure]. Madison, WI: University of Wisconsin-Extension.
University of Wisconsin-Extension. (2003). A string of pearls (2nd ed.) [Video]. Superior, WI: Author.
Wisconsin Natural Heritage Inventory Program. (2006). Wisconsin Natural Heritage working list. Madison, WI: Wisconsin Department of Natural Resources.