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The Yuba Goldfields is a valley of 10,000 acres (40 km2) on both sides of the Yuba River in Yuba County, California, located northeast of Marysville. The goldfields are noted for their otherworldly appearance, filled with oddly shaped gravel mountains, ravines, streams and turquoise-colored pools of water. From the air, the goldfields are said to resemble intestines. Wild turkeys, deer, ducks, Beavers, herons, bald eagles, Northern river otters and even mountain lions now call the goldfields home. ==History== They were created during the California Gold Rush. The first Yuba-area miners panned for gold in stream beds in the valley but within a decade large-scale industrial processes replaced solitary prospectors. Mining companies moved from the valley floor into the Sierra Nevada foothills, where miners blasted gravel hillsides with high-pressure jets of water—a process called hydraulic mining. After the miners extracted gold in long wooden sluices, they dumped the gravel back into the mountain valleys. Rivers and streams carried the flood of sediment—called slickens—down to the Sacramento Valley. Upwards of of debris was deposited in the Yuba River. The mine waste carried by the Yuba River ended up raising the riverbed (by up to in some cases), causing floods that buried farms east of the town of Marysville with gravel, mud, as well as mercury and arsenic (byproducts of the mining process). As the Yuba River is a tributary of the Sacramento River, much of that debris then found its way to the San Francisco Bay. In Sacramento, the I Street Bridge had to be raised twenty feet (6 m). Lawsuits by farmers curtailed hydraulic mining in 1883, but the gravel remained behind in the Yuba Goldfields.〔(Yuba Goldfields ) at NASA Earth Observatory. This article incorporates public domain text from this US Government website.〕 In 1893, the California Debris Commission began to dredge the Yuba near Marysville to mitigate the environmental damage, and piled the gravel along the river's banks, creating the irregular hills seen today. The dredgers also created over 200 ponds, which are fed by a network of underground rivers, which in turn were formed due to the porosity of the ground. The water in these ponds is usually clear blue, the impurities having been filtered by the gravel.〔 In the twentieth century, a series of mining companies reprocessed the slickens, extracting gold that was increasingly difficult to separate from the gravel. Even though the ore had already been processed, it was the principal source of gold in California for some time. By the 1970s, it was economically impossible to retrieve any more gold, and the debris became a source of aggregate, an essential ingredient of concrete. It has been estimated that up to $15 billion worth of aggregate lies within the goldfields, although the mining industry disputes this figure. The environmental damage caused by the dredgers is enormous. The top of many square miles was turned upside-down. The soil and the rocks were separated with the rocks stacked on top of the soil. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yuba Goldfields」の詳細全文を読む スポンサード リンク
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