Johnstown prospered from skyrocketing demand in the western United States for barbed wire. Through the second half of the 19th century, Johnstown made much of the nation's barbed wire. By 1860, the Cambria Iron Company of Johnstown was the leading steel producer in the United States, outproducing steel giants in Pittsburgh and Cleveland. Iron, coal, and steel quickly became central to the town of Johnstown. The railroads provided large-scale development of the region's mineral wealth. Johnstown became a stop on the main line of the Pennsylvania Railroad and was connected with the Baltimore & Ohio. With the coming of the railroads, the city's growth improved.

By 1854, canal transport became redundant with the completion of the Pennsylvania Railroad, which now spanned the state. Perhaps the most famous passenger who traveled via the canal to visit Johnstown briefly was Charles Dickens in 1842. Johnstown was at the head of the canal's western branch, with canal boats having been transported over the mountains via the Allegheny Portage Railroad and refloated here, to continue the trip by water to Pittsburgh and the Ohio Valley. From 1834 to 1854, the city was a port and key transfer point along the Pennsylvania Main Line Canal. The community incorporated as Conemaugh borough January 12, 1831, but renamed Johnstown on April 14, 1834. The settlement was initially known as 'Schantzstadt', but was soon anglicized to Johnstown. Johnstown was formally platted as Conemaugh Old Town in 1800 by the Swiss German immigrant Joseph Johns (born Josef Schantz). 1790s ДЄ settlement was established here in 1791 by Joseph Jahns, in whose honor it was named, and the place was soon laid out as a town. Individual listings on the National Register of Historic Places are the Grand Army of the Republic Hall, Cambria Iron Company, Cambria Public Library Building, Bridge in Johnstown City, Nathan's Department Store, and Johnstown Inclined Railway. The city is home to five national historic districts: the Downtown Johnstown Historic District, Cambria City Historic District, Minersville Historic District, Moxham Historic District, and Old Conemaugh Borough Historic District. Д«efore becoming an independent town, Windber, Pennsylvania, was considered a part of the city. The 1977 flood-in what was to have been a 'flood-free' city-may have contributed to Johnstown's subsequent population decline and inability to attract new residents and businesses. Despite a pledge by President Franklin Delano Roosevelt to make the city flood free, and subsequent work to do so, another major flood occurred in 1977. At least 2,209 people died as a result of the flood and subsequent fire that raged through the debris. The Johnstown Flood of May 31, 1889, occurred after the South Fork Dam collapsed 14.1 miles (22.7В km) upstream from the city during heavy rains.

Johnstown, settled in 1770, has experienced three major floods in its history.

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