How climate change is affecting freshwater lakes

by Abby Simpson

Increased water temperatures, decreased ice cover, droughts, wildfires, floods, and stronger weather patterns caused by climate change are having a detrimental effect on the United States’ freshwater lakes and reservoirs. We must take urgent action now before the negative effects become irreversible.

The U.S. is home to about 250 massive freshwater lakes with surface areas of 10 square miles or greater,1 as well as tens of thousands of smaller lakes and more than 1,900 reservoirs.2 Michigan alone has more than 11,000 freshwater lakes3 and 225 reservoirs.4 These lakes, natural and man-made, are vital components of the ecosystem’s health and support the lives of plant and animal species, as well as humans. The Great Lakes alone hold 90% of the United States’ fresh water supply.5

Climate Change Affects Regions Differently

Warmer temperatures in the Great Lakes region, which includes Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania, and Wisconsin, have contributed to decreased water levels by increasing evaporation rates.6 An increase in water temperature causes ice to form later than usual, allowing for extended periods of evaporation, which also reduces water levels. In addition, longer warming seasons have led to prolonged separation of water into layers — warmer, lighter water on top, and cooler, denser water below. The difference between these two layers can prevent oxygen from the surface from sinking deeper into the lake (stratification). As a result, fish and other oxygen-consuming organisms will either attempt to move on or suffocate.

In addition, warmer temperatures and stratified lakes create optimal conditions for algae to grow and cause oxygen-depleted dead zones. Algal blooms are toxic and not only deadly to aquatic life but land animals and humans as well. For example, the Toledo water crisis, caused by toxic blue green algae that infiltrated the city’s municipal water supply, caused more than half a million Ohio residents to be without drinking or bathing water.7 These blooms cause mass die offs of fish and aquatic species. Freshwater algal blooms have increased significantly over the past 40 years as atmospheric temperatures continue to increase.8 These blooms now pose an environmental problem in all 50 states, and this will continue to worsen, putting both animals and humans in grave danger.9 Predators that once preyed on these now absent organisms will be forced to find other sources of food. This means they will either find new species in the area to prey upon, leave the area in search of food, or starve to death.

Extreme heat and heat waves are causing droughts in the western U.S., posing a threat to the region’s freshwater supplies.10 Nearly 93% of the area is experiencing drought or abnormally dry conditions, and more than 70% is experiencing severe or extreme drought conditions.11 The region has been in a 22-year megadrought with reservoirs, such as Lake Mead, suffering tremendously.12 Lake Mead is currently filled to 27% of its capacity,13 relying on about 10% of its water from local precipitation, groundwater, and snow melt.14 As temperatures increase in the winter and spring, the early run off of rainwater and snow melt grows. This leads to a decline in run off by summer, which, in conjunction with dry and hot conditions, causes increasingly destructive fires. As these fires burn through thousands, even millions of acres, they leave behind a denuded landscape. Subsequent rainfall and snow melt then causes the burnt debris remnants to wash into lakes resulting in increased sediment and contaminant load. The absence of vegetation then increases rates of erosion and flooding when rainfall occurs.15 These effects also place a heavy burden on infrastructure, including water treatment plants.

Even as climate change worsens droughts, it also increases the likelihood of stronger storms which, in certain areas of the U.S., has increased precipitation. As atmospheric temperatures increase, so do rates of evaporation. Increased evaporation leads to more moisture in the atmosphere, which eventually drops in the form of flash floods, hurricanes, heavy snows, and other extreme weather events.16 This heightened precipitation can increase water levels, flooding, and erosion.17 Since 1951, the Great Lakes region has experienced a 14% increase in precipitation. Across the Great Lakes region, “the amount of precipitation falling during the heaviest 1% of precipitation days increased by 35% from 1951 to 2017.”18 For example, in June 2021, up to 7 inches of rainfall occurred in less than 24 hours in the Detroit metro area.19 This intense amount of rainfall and runoff poses threats of erosion. Fluctuation in water levels, heavy rainfall, surface water runoff, and wave action have begun eroding the Great Lake’s vital shoreline. Currently, about 250 miles of shoreline along lakes Michigan, Superior, and Huron are designated as high risk of erosion,20 and the impacted area is expected to increase as atmospheric temperatures and heavy rainfall surge.

No matter the region, surging atmospheric temperatures will have an increasingly damaging effect on our freshwater lakes. Animals and humans will suffer from the effects of increased algal blooms, droughts, fires, heightened storms, and flooding. Increased rates of runoff in winter and spring and burnt debris left from ravaging wildfires pose a strong threat to freshwater sources in the Western U.S. And run off, sediment load, and erosion caused by heavy rainfall sweeping the Great Lakes Region threaten the area’s freshwater supply.

Abby Simpson is a contributing member of the Grassroots Network Climate Emergency Mobilization team. If you have a suggestion for a future topic or are interested in joining the team, please reach out to us at climateemergency[at]sfbaysc[dot]org.


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