If you live hundreds of miles away from the coast, it probably never occurs to you that the plastic bag or cup lid that you toss into the gutter might make its way into the Atlantic or the Pacific. But somewhere between 40,000 and 110,000 metric tons of plastic waste generated by Americans ends up in the ocean, according to a groundbreaking study published earlier this year in the journal Science.

It's difficult to pinpoint where all that refuse originates, and researchers think that much or most of it probably comes from the nation's densely-populated coastlines. But there's also evidence that the nation's inland waterways serve as a conduit for plastic to travel thousands of miles into the oceans.

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"The ocean is always downstream," explained Nicholas Mallos, a conservation biologist and ocean debris expert for the Washington, DC-based Ocean Conservancy.

When the conservancy has staged cleanup campaigns along inland waterways far from the ocean, they found large amounts of plastic in rivers and lakes, Mallos said. In addition to littering, "We've often seen illegal dumping of trash in rural areas, and that includes a lot of plastic." A 2009 effort by the conservancy found aquatic waste, much of it plastic items, in 45 states and the District of Columbia.

While researchers have documented the vast floating gyres of plastics and other human trash floating in the world's oceans, there's been relatively little attention paid to plastic in rivers, streams and lakes.

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"To my knowledge no one has studied particular routes, with the exception of places like L.A. and Baltimore Harbor where there are trash containment measures in place to prevent debris in rivers from entering the ocean," said Kara Lavender Law, an oceanographer who is principal researcher for the Sea Education Association. "But we don't know what fraction of this waste originated close to the coast versus far inland."

The few studies that exist, however, suggest that it may be a huge problem. A 2011 study of two southern California urban rivers found that that every square meter of water contained from 125 to 819 pieces larger than 4.75 millimeters.  A 2013 survey of the Meuse River, which flows 575 miles through France, Belgium and the Netherlands to the North Sea, found that it contained 70,000 pieces of plastic per square meter of water, about 500 of which were roughly an inch or bigger in size.

The studies indicate that most of the plastic pollution in rivers consists of microplastics -- tiny fragments that form when big pieces of plastic waste are exposed to sunlight. Microplastic is easily ingested by marine life, both in rivers and eventually in the ocean. A study published in June found that even microscopic zooplankton, which are a food source for ocean animals, are consuming microplastics at an alarming rate.

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Besides plastic trash, recent research suggests that lakes and rivers are polluted with an even more insidious form of plastic -- tiny strands of petroleum-based fabrics such as polyester and nylon, some just a fraction of a millimeter in diameter, which are shed by clothing made from synthetic fabrics when it is laundered. Because the fibers are so tiny, they are washed down drains into sewers and slip through filtering systems in wastewater management plants.

Researchers who used fine mesh nets to strain water from Lake Michigan found more than 19,000 strands per square kilometer during a yet-unpublished 2013 study, the Chicago Tribune reported in February. Scientists believe that the fibers are swallowed by fish and other marine animals, and probably become lodged in their bodies, along with toxic chemicals and bacteria that have been absorbed by the fibers.

If there's any positive takeaway in this, it's that you can do something, at least on a personal level, to reduce the amount of plastic that goes into the oceans. "Put trash and recycling where it goes," said Jenna Jambeck, an associate professor of environmental engineering at the University of Georgia. "Use reusable items -- bags, cups and bottles -- to reduce waste generation."

Finally, Jambeck urges people to pick up litter along or in waterways, and log it with a phone app called the Marine Debris Tracker. The data your provide can help scientists to get a better handle on the aquatic trash problem.