A garbage patch is a gyre of marine debris particles caused by the effects of ocean currents and increasing plastic pollution by human populations. These human-caused collections of plastic and other debris, cause ecosystem and environmental problems that affect marine life, contaminate oceans with toxic chemicals, and contribute to greenhouse gas emissions. Once waterborne, marine debris becomes mobile. Flotsam can be blown by the wind, or follow the flow of ocean currents, often ending up in the middle of oceanic gyres where currents are weakest. Garbage patches grow because of widespread loss of plastic from human trash collection systems.
The United Nations Environmental Program estimated that "for every square mile of ocean" there are about "46,000 pieces of plastic." The 10 largest emitters of oceanic plastic pollution worldwide are, from the most to the least, China, Indonesia, Philippines, Vietnam, Sri Lanka, Thailand, Egypt, Malaysia, Nigeria, and Bangladesh, largely through the rivers Yangtze, Indus, Yellow, Hai, Nile, Ganges, Pearl, Amur, Niger, and the Mekong, and accounting for "90 percent of all the plastic that reaches the world's oceans." Asia was the leading source of mismanaged plastic waste, with China alone accounting for 2.4 million metric tons.
The best known of these is the Great Pacific garbage patch which has the highest density of marine debris and plastic. Other identified patches include the North Atlantic garbage patch between North America and Africa, the South Atlantic garbage patch located between eastern South America and the tip of Africa, the South Pacific garbage patch located west of South America, and the Indian Ocean garbage patch found east of south Africa listed in order of decreasing size. In the Pacific Gyre, specifically 20°N-40°N latitude, large bodies with floating marine debris can be found. Models of wind patterns and ocean currents indicate that the plastic waste in the northern Pacific is particularly dense where the Subtropical Convergence Zone (STCZ), meets a southwest–northeast line, found north of the Hawaiian archipelago.
In the Pacific, there are two mass buildups: the western garbage patch and the eastern garbage patch, the former off the coast of Japan and the latter between Hawaii and California. The two garbage patches are both part of the great Pacific garbage patch, and are connected through a section of plastic debris off the northern coast of the Hawaiian islands. These garbage patches contain 90 million tonnes (100 million short tons) of debris. The waste is not compact, and although most of it is near the surface of the pacific, it can be found up to more than 30 metres (100 ft) deep in the water.
2017 research  reported "the highest density of plastic rubbish anywhere in the world" on remote and uninhabited Henderson Island in South Pacific as a result of the South Pacific Gyre. The beaches contain an estimated 37.7 million items of debris together weighing 17.6 tonnes. In a study transect on North Beach, each day 17 to 268 new items washed up on a 10-metre section.
In 2014, there were five areas across all the oceans where the majority of plastic concentrated. Researchers collected a total of 3070 samples across the world to identify hot spots of surface level plastic pollution. The pattern of distribution closely mirrored models of oceanic currents with the North Pacific Gyre, or Great Pacific Garbage Patch, being the highest density of plastic accumulation. The other four garbage patches include the North Atlantic garbage patch between the North America and Africa, the South Atlantic garbage patch located between eastern South America and the tip of Africa, the South Pacific garbage patch located west of South America, and the Indian Ocean garbage patch found east of south Africa.
The Great Pacific garbage patch (also Pacific trash vortex) is a garbage patch, a gyre of marine debris particles, in the central North Pacific Ocean. It is located roughly from 135°W to 155°W and 35°N to 42°N. The collection of plastic and floating trash originates from the Pacific Rim, including countries in Asia, North America, and South America. The gyre is divided into two areas, the "Eastern Garbage Patch" from California to Hawaii, and the "Western Garbage Patch" extending from Hawaii to Japan.
Despite the common public perception of the patch existing as giant islands of floating garbage, its low density (4 particles per cubic metre (3.1/cu yd)) prevents detection by satellite imagery, or even by casual boaters or divers in the area. This is because the patch is a widely dispersed area consisting primarily of suspended "fingernail-sized or smaller"—often microscopic—particles in the upper water column known as microplastics. Researchers from The Ocean Cleanup project claimed that the patch covers 1.6 million square kilometres (620 thousand square miles). Some of the plastic in the patch is over 50 years old, and includes items (and fragments of items) such as "plastic lighters, toothbrushes, water bottles, pens, baby bottles, cell phones, plastic bags, and nurdles." The small fibers of wood pulp found throughout the patch are "believed to originate from the thousands of tons of toilet paper flushed into the oceans daily."Research indicates that the patch is rapidly accumulating. The patch is believed to have increased "10-fold each decade" since 1945. Estimated to be double the size of Texas, the area contains more than 3 million short tons (2.7 million metric tons) of plastic. The gyre contains approximately six pounds of plastic for every pound of plankton. A similar patch of floating plastic debris is found in the Atlantic Ocean, called the North Atlantic garbage patch. This growing patch contributes to other environmental damage to marine ecosystems and species.
The 10 top-ranked rivers transport 88–95% of the global load into the sea
It turns out that about 90 percent of all the plastic that reaches the world's oceans gets flushed through just 10 rivers: The Yangtze, the Indus, Yellow River, Hai River, the Nile, the Ganges, Pearl River, Amur River, the Niger, and the Mekong (in that order).