The western end of Lake Erie now has a greenish tint, recent satellite images show. Close-ups show the area between northwest Ohio, southeast Michigan and the far southwestern portion of Ontario.

When heavy rains hit, municipal sewer systems are often overwhelmed, dumping a mix of storm water and untreated sewage into nearby waterways.

It's a nationwide problem — an estimated 775 communities in the United States have sewer systems constructed this way, with combined sewage and storm water pipes, including New York City and Chicago. Combined sewer overflows, as these events are known, can have far-reaching impacts. For example, CSO outfalls are "the single largest source of pathogens to the New York Harbor system, according to the New York Department of Environmental Protection," Newsweek recently reported.

(ABOVE: Aerial Views of Lake Erie Algae)

In the Great Lakes region, CSOs contribute to the growth of the toxic algae blooms that recently cut off the water supply in Toldeo, Ohio, for nearly 48 hours, thanks to high levels of the toxin microcystin.

Here's how: Blue-green algae feed off of phosphorus. Frequently, agricultural runoff is fingered as the top cause of phosphorus in fresh waterways. But the nutrient comes from sewage, too.

On Thursday, the Michigan Department of Environmental Quality introduced new programs to reduce the amount of phosphorus Michigan continues to Lake Erie, including a goal to improve phosphorus removal at Detroit metro-area wastewater treatment plants, The Detroit News reported Friday, days after a six-inch deluge sent a spout of fertilizers, chemicals and wastewater into the Detroit River, which flows into Lake Erie.

Still, the phosphorus problem might get worse before it gets better. Nutrient enrichment and climate change cause "an apparent increase in the toxicity of some algal blooms in freshwater lakes and estuaries around the world,"Oregon State University scientistssaid in 2013 in the journalScience. Rising temperatures and carbon dioxide concentrations contribute to the problem.

Although not all algae blooms pose a human health threat (microcystin and other toxins are not always present), they do deprive fish and other aquatic life of oxygen, sometimes creating "dead zones" where life cannot thrive. In the Gulf of Mexico, one of the world's most famous oxygen-starved hypoxic zones changes size each year based on weather patterns. But under the right conditions, it could grow to the size of the state of New Jersey, scientists from the University of Michigan reported in 2013.

As for Detroit's recent rainfall and the resulting sewer outfall, “this should be a wake-up call,” Melissa Damaschke, of Michigan's Sierra Club, told The Detroit News. “These extreme weather events are happening and we need to address them.”

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