on Sulphur Creek. The Sulphur Creek Dam, completed in 1966, was built as an irrigation reservoir, but was acquired by the Orange County Flood Control District in 1970. In addition, there are two detention basins on the main stem of Aliso Creek. The first is El Toro Detention Basin, an off-stream basin in a portion of Heroes Park in Lake Forest, and the second is the on-stream Pacific Park Detention Basin in Aliso Viejo. There are also detention basins constructed on upper Wood Canyon Creek, Dairy Fork, and English Canyon Creek. Nineteen drop structures have been constructed on Aliso Creek to mitigate the damaging erosion caused by increased flooding. Many of these structures were constructed in response to a 1969 flood that caused $1 million of damage along Aliso Creek, while others were necessary to mitigate erosion caused by artificial straightening of the stream channel. In one of the largest such projects, at Aliso Creek Road, the creek was realigned in an excavated channel through a ridge in order to accommodate the construction of the Chet Holifield Federal Building and Alicia Parkway. Because this shortened the stream channel by about and increased its gradient, two drop structures were constructed to prevent the creek from eroding upstream. Environmental issues Aliso Creek is listed as a Clean Water Act impaired water, which is defined by the U.S. Environmental Protection Agency as "impaired by one or more pollutants that do not meet one or more water quality standards". The creek frequently exceeds bacterial limits set by California law. At Aliso Beach, the creek water is considered unsafe for swimming 99 percent of the time. Contact with seawater at the beach is discouraged for 72 hours (3 days) after a major storm event. The creek contains elevated levels of E. coli, phosphorus, nitrogen and selenium, which comes mainly from heavy application of fertilizer and manure, and other organic pollutants in urban runoff. High water temperatures, sometimes exceeding in the summer, promote the growth of bacteria as well as algae blooms leading to eutrophication. Runoff of chlorine, oils and heavy metals, mainly caused by irrigation and car washing, have killed off most fish and shrimp species in the creek, with the exception of carp. The Los Angeles Times reported in 1997 that "County health officials acknowledge that the bacterial count at the mouth of the creek—which curls into a warm-water stagnant pond that flushes out