<P> A dead zone exists in the central part of Lake Erie from east of Point Pelee to Long Point and stretches to shores in Canada and the United States . The zone has been noticed since the 1950s to 1960s, but efforts since the 1970s have been made by Canada and the US to reduce runoff pollution into the lake as means to reverse the dead zone growth . Overall the lake's oxygen level is poor with only a small area to the east of Long Point that has better levels . The biggest impact of the poor oxygen levels is to lacustrine life and fisheries industry . </P> <P> A dead zone exists in the Lower St. Lawrence River area from east the Saguenay River to east of Baie Comeau, greatest at depths over 275 metres (902 ft) and noticed since the 1930s . The main concerns for Canadian scientists is the impact of fish found in the area . </P> <P> Off the coast of Cape Perpetua, Oregon, there is also a dead zone with a 2006 reported size of 300 square miles (780 km2). This dead zone only exists during the summer, perhaps due to wind patterns . The Oregon coast has also seen hypoxic water transporting itself from the continental shelf to the coastal embayments . This has seemed to cause intensity in several areas of Oregon's climate such as upwelled water containing oxygen concentration and upwelled winds . </P> <P> The area of temporary hypoxic bottom water that occurs most summers off the coast of Louisiana in the Gulf of Mexico is the largest recurring hypoxic zone in the United States . The Mississippi River, which is the drainage area for 41% of the continental United States, dumps high - nutrient runoff such as nitrogen and phosphorus into the Gulf of Mexico . According to a 2009 fact sheet created by NOAA, "seventy percent of nutrient loads that cause hypoxia are a result of this vast drainage basin". which includes the heart of U.S. agribusiness, the Midwest . The discharge of treated sewage from urban areas (pop . c 12 million in 2009) combined with agricultural runoff deliver c. 1.7 million tons of phosphorus and nitrogen into the Gulf of Mexico every year . Even though Iowa occupies less than 5% of the Mississippi River drainage basin, average annual nitrate discharge from surface water in Iowa is about 204,000 to 222,000 metric tonnes. or 25% of all the nitrate which the Mississippi River delivers to the Gulf of Mexico . Export from the Raccoon River Watershed is among the highest in the United States with annual yields at 26.1 kg / ha / year which ranked as the highest loss of nitrate out of 42 Mississippi subwatersheds evaluated for a Gulf of Mexico hypoxia report . </P>

When did the gulf of mexico dead zone happen