The experience changed Moore's life, turning him from an adventurer into a self-taught scientist and environmental activist.
Two years later, he returned to the garbage patch with a volunteer crew to survey its contents. He knew he would collect plenty of plastic bags, bottle caps, nets and floats.
He didn't expect what turned up in a special net, one with a tight mesh for collecting plankton, the bottom link in the oceanic food chain. Instead of plankton, it was choked with a colorful array of tiny plastic fragments.
"It blew my mind," Moore said. "We are filling up the oceans with this confetti stuff, and nobody cares."
Over the last decade, Moore, 59, who lives in a waterfront home in Long Beach, has spent his own money and some from a family foundation on a quest to track the plume of plastic so he can figure out how to stop it.
On a cloudless spring day, Moore waded up to his knees into the Los Angeles River in Long Beach wearing shorts, sandals and a white hard hat. He was tethered to a volunteer standing on the dry riverbank, in case he slipped on the slick concrete channel.
The Los Angeles River carries enough trash each year to fill the Rose Bowl two stories high, and despite efforts to corral some of it near the river mouth, most slips through to the ocean.
Moore adjusted a trawlnet to collect trash flowing downriver. At Moore's signal, a crane operator lifted the net out of the water. Volunteers swarmed around the trawlnet, extracted the contents and loaded them into more than a dozen jars.
The jars were filled with plastic pellets the size and shape of pills. They come in all colors and are the raw material for a vast array of plastic products, from trash bags to medical devices.
About 100 billion pounds of pellets are produced every year and shipped to Los Angeles and other manufacturing centers. Huge numbers are spilled on the ground and swept by rainfall into gutters; down storm drains, creeks and rivers; and into the ocean.
From his river sampling, Moore estimated that 236 million pellets washed down the Los Angeles and San Gabriel rivers in three days' time. Also known as "nurdles" or mermaid tears, they are the most widely seen plastic debris around the world. They have washed ashore as far away as Antarctica.
The pellets, like most types of plastic, are sponges for oily toxic chemicals that don't readily dissolve in water, such as the pesticide DDT and polychlorinated biphenyls, or PCBs. Some pellets have been found to contain concentrations of these pollutants 1 million times greater than the levels found in surrounding water.
As they absorb toxic chemicals, they become poison pills. Wildlife researchers have found the pellets, which resemble fish eggs, in the bellies of fish, sea turtles, seabirds and marine mammals.
Over time, plastic can break down into smaller and smaller pieces, eventually turning to powder and entering the ocean in microscopic fragments. Some plastic starts out as tiny particles, such as the abrasives in cleaning products that are washed down the sink, through sewage systems and out to sea.
The chemical components of plastics and common additives can harm animals and humans. Studies have linked the hormone-mimicking phthalates, used to soften plastic, to reduced testosterone and fertility in laboratory animals, and to subtle changes in the genitals of baby boys. Another additive, bisphenol A, used to make lightweight, heat-resistant baby bottles and microwave cookware, has been linked to prostate cancer.
Moore has tried, without success, to get manufacturers to improve their efforts to clean up spills of pellets that wash off lots and into storm drains. He considers beach cleanups a waste of time, except to raise public awareness of the problem. In his view, the cleanup has to start at the source — many miles inland.
To make that point, Moore tromped through rail yards in Vernon and La Mirada. On the side of a rail car a faded decal read "Operation Clean Sweep." It had three check boxes:
"Keep Plastics Off Ground.
"Close and Lock Caps When Outlets Not in Use.
"Pick Up All Spills."
Beneath the sign was a cone-shaped pile of pellets, as white as freshly fallen snow. Moore shuffled his sandaled feet through another drift nearby.
"This is a plastic sand dune," he said. "It's very slippery, very roly-poly. What makes them so good for the factory makes them good for getting into the ocean."
Times staff writer Usha Lee McFarling contributed to this report.