river in Lowell is serious class 3+/4 whitewater. Dams cause fish population to decrease Dams were built along the Concord River to increase crop production and also to provide a source of power for operating mills. By the 19th century, the native population of shad and alewife became extinct, because the dams prevented the mature fish from returning upstream to spawn. Alewife and other anadromous fish are migratory. They hatch in freshwater, make their way to the sea to grow, then return as adults to freshwater to spawn, usually near where they had hatched. This instinct is imprinted within the fish when it is born. So, when the route upstream became blocked, the cycle was broken. The Faulkner Dam in North Billerica is just one of many blockages that caused the alewife population to collapse on the Concord River. At this point, water was diverted north to Lowell and south to Charlestown to run the Middlesex Canal. Effects of pollution During the 19th century, the Concord River was near the heart of the US Industrial Revolution. Textile, paper, tanning, and mining industries all emerged during this period. Industrial wastes were dumped into the river, along with untreated sewage and other organic waste. Industrial development within the watershed peaked in the 1920s. The early 1960s saw an increase in the number of chemical and metal firms, many supporting the electronics industry within the Boston metropolitan area. By the 1960s, the Merrimack River was considered one of the top ten most polluted waterways in America. The Concord River empties into the Merrimack River. The passage of the Clean Water Act in 1972 signaled the beginning of US efforts to improve the quality of its waterways. The act imposed stricter controls on point source discharges into rivers and other navigable waters. This eventually led to the installation of three wastewater treatment plants on the banks of the Concord River: one in Concord and two in Billerica. These plants have helped to prevent subsequent damage to the river ecosystem when they were able to operate within federally mandated limits. Pollutants such as heavy metals and PCBs remain trapped in the sediment. Throughout much of the Sudbury River and downstream into the Concord River, fish consumption is banned due to mercury-laden sediments from the Nyanza Superfund site and other sources. Perchlorate was detected in the Concord River in August 2004. Initially, it was believed that