<P> However contrary to federal laws in both countries, those treatment system upgrades have not yet eliminated Combined sewer Overflow events . This describes when older sewerage systems, which combine storm water with sewage into single sewers heading to the treatment plant, are temporarily overwhelmed by heavy rainstorms . Local sewage treatment authorities then must release untreated effluent, a mix of rainwater and sewage, into local water bodies . While enormous public investments such as the Deep Tunnel projects in Chicago and Milwaukee have greatly reduced the frequency and volume of these events, they have not been eliminated . The number of such overflow events in Ontario, for example, is flat according to the International Joint Commission . Reports about this issue on the U.S. side highlight five large municipal systems (those of Detroit, Cleveland, Buffalo, Milwaukee and Gary) as being the largest current periodic sources of untreated discharges into the Great Lakes . </P> <P> Phosphate detergents were historically a major source of nutrient to the Great Lakes algae blooms in particular in the warmer and shallower portions of the system such as Lake Erie, Saginaw Bay, Green Bay, and the southernmost portion of Lake Michigan . By the mid-1980s, most jurisdictions bordering the Great Lakes had controlled phosphate detergents, resulting in sharp reductions in the frequency and extent of the blooms . </P> <P> In 2013, news of a garbage patch of plastic pollution in the lakes was reported . </P> <P> Several Native American tribes inhabited the region since at least 1000 BC, after the end of the Wisconsin glaciation . The peoples of the Great Lakes traded with the Hopewell culture from around 1000 AD, as copper nuggets have been extracted from the region, and fashioned into ornaments and weapons in the mounds of Southern Ohio . The brigantine Le Griffon, which was commissioned by René - Robert Cavelier, Sieur de La Salle, was built at Cayuga Creek, near the southern end of the Niagara River, and became the first known sailing ship to travel the upper Great Lakes on August 7, 1679 . </P>

What nation contains nearly half of the worlds lakes