back to the ocean. On May 14, 2007, onlookers and media spotted two humpback whales traveling the deep waters near Rio Vista. The duo, generally believed to be mother and calf (Delta, the mother and Dawn, her calf), continued to swim upstream to the deep water ship channel near West Sacramento, about inland. There was concern because the whales had been injured, perhaps by a boat's propeller or keel, leaving a gash in each whale's skin. The whales were carefully inspected by biologists and injected with antibiotics to help prevent infection. After days of efforts to lure (or frighten) the whales in the direction of the ocean, the whales eventually made their way south into San Francisco Bay, where they lingered for several days. By May 30, 2007, the cow and calf apparently slipped out unnoticed under the Golden Gate Bridge into the Pacific Ocean, likely under cover of night. Pollution For a river of its size, the Sacramento is considered to have fairly clean water. However, pollutants still flow into the river from many of its tributaries and man-made drains or channels. Pesticide runoff, especially DDT, is one of the largest problems faced today, because of the valley's primarily agricultural economy. Increased erosion caused by the removal of riparian vegetation and the runoff of fertilizers into the river have led to occasional algae blooms, though the water is usually cold because of the regulation of dams upstream. Other pollutant sources include urban runoff, mercury and even rocket fuel that was reported to have leaked near the American River from an Aerojet extraction project. Mercury pollution created by mining and processing activities during the California Gold Rush still has a profound impact on the Sacramento River's environment. According to Domagalski of the USGS, Mercury is currently considered the most serious water-quality problem in the Sacramento River. Mercury is unique because it is the heaviest liquid in existence, it is the only heavy metal that is a liquid at room temperature and it readily vaporizes into the atmosphere. The toxic substance was widely used by miners to separate gold from the surrounding rocks and dirt, and was disposed of by allowing it to evaporate. Most of the mercury was mined in the Coast Ranges to the west of the Sacramento River; mines in these mountains produced roughly 140,000 tons of mercury to serve the Gold Rush. When the gold rush ended,