The Yangtze is Asia's longest river and the third-longest river in the world. It also tops the list of river systems through which the most plastic waste flows into the oceans, according to a recent study. The Yangtze flows into the East China Sea near Shanghai and is crucial to China's economy and ecology. The river basin is home to 480 million people — one-third of the country's population.

The Helmholtz Centre for Environmental Research found 90 percent of plastic flowing into oceans can be traced to 10 rivers. The Indus ranks second on the list. One of Asia's largest rivers, it flows through parts of India and Pakistan into the Arabian Sea, supporting millions of people. While much plastic enters rivers because of a lack of waste infrastructure, sewage systems contribute too.

Plastic can enter the food chain as fish and other marine and freshwater animals ingest it. The Yellow River, said to be the cradle of Chinese civilization, is third on the plastic-waste list but that's not the only environmental problem with which it contends. Pollution has rendered much of the river's water undrinkable. Around 30 percent of its fish species are believed to have disappeared too.

Another of China's rivers, the Hai, comes in at number 4. It connects two of China's most populous metropolitan areas, Tianjin and Beijing, before flowing into one of the world's busiest shipping lanes, the Bohai Sea. The 10 river systems share traits, says the study. One is that they are located in densely populated areas with a lack of waste infrastructure and little awareness of recycling.

Generally thought to be the world's longest river, the Nile flows through 11 countries before entering the Mediterranean Sea from Egypt. Some 360 million people live in the river basin where its waters support agriculture — the region's main economic activity. Irrigation and evaporation mean the river doesn't even reach the sea in dry periods. Still, it comes in at number five in the ranking.

The Ganges is central to Indian spiritual life and provides water to more than half a billion people. Sewage, agricultural and industrial waste have made it one of the world's most polluted rivers, as have the multitudes of plastic that end up in it. Cleaning up the waste — as students are doing in this picture — is important, but experts say we must produce less and stop pollution at the source.

Here, workers clear floating waste from China's notoriously dirty Pearl River, which enters the South China Sea between Hong Kong and Macau. Sewage and industrial waste flow into the river delta, keeping apace with the region's incredible rate of urban expansion. Since the late 1970s, the delta has transformed from a mainly agricultural and rural region to one of the world's largest urban areas.

It's not until they hit urban and industrial areas that rivers feel the worst effects of pollution. Still, according to recent studies, plastic debris is even being found in remote and "pristine" locations. The Amur River rises in the hills of northeastern China and forms much of the border between China's Heilongjiang province and Russia's Siberia before it snakes out to the Sea of Okhotsk.

The Niger is West Africa's main river, supporting over 100 million people and one of the planet's most lush ecosystems. It flows through five countries before entering the Atlantic Ocean from Nigeria. Plastic pollution aside, extensive dam construction is affecting water availability — and frequent oil spills in the Niger Delta have caused widespread water contamination.

Dams are having major ecological and social impacts on the Mekong too. Around 20 million people live in the Mekong Delta. Many are dependent on fishing and agriculture for survival. The river flows through six countries in Southeast Asia, including Vietnam and Laos, and is tenth on the list of river systems that carry most of the 8 million tons of plastic that are dumped into the seas each year.