The California gold rush left a legacy of pollution that could impact the west coast for thousands of years, USA Today reported.

Most historians thought the last of dangerous mercury byproduct created by environmentally catastrophic mining techniques either ran off into the San Francisco Bay by the early 1900s or has remained in the Sierra Nevada's, according to USA Today. Recent research by geomorphologist Michael Singer and colleagues, however, discovered fresh mercury deposits in the valleys below old gold mines, signifying the compound has escaped the mountains.

Once set free in the valleys, mercury has the potential to enter the food chain and leak into the San Francisco Bay, USA Today reported. Unsafe mercury can cause neurological disorders. The levels are significant, according to Singer.

"There are massive amounts of mercury-laden sediment making its way down to the lowlands," he said.

To extract gold, ambitious miners in the 1850's used a technique called hydraulic mining, USA Today reported. First, they blasted mountain sides and created a mixture of gold, gravel and sand. Then, they separated the gold by dropping in hundreds of pounds of mercury.

Singer and his group conducted their search on 105 sites along the Yuba River, which flows from the Sierra's and into the lowlands. Several old gold mines are positioned along the river. They found relatively recent deposits throughout the sites, including an area known as the Yuba Fan, a site along the river described as a "gigantic buildup of mining debris" by USA Today.

Researchers also discovered that the most significant mercury deposits occurred as a result of major floods in 1986, 1987, and 2006, according to USA Today. If climate change creates more flooding, as it is expected to do, more mercury will wash away from the mountain and down the Yuba River, Singer said.

His team didn't explore the path of mercury beyond the Yuba and into the San Francisco Bay, but there's evidence to support that it has and will continue to follow that path, according to Duke University geochemist Gretchen Gehrke, who was not involved in the study.

There is a "romantic view of the Gold Rush, which is old guys roaming around with pans of gold. It was really an industrialized operation run by engineers," said Singer. But the region has a "problem ... much, much bigger than (many others) are suspecting it is."

The report will publish this week in Proceedings of the National Academy of Sciences.