Formed more than 15,000 years ago by glacial flow, the Nashua River shaped Nashua's industrial history and remained important to transportation, leisure, and community life, serving as the town skating rink and the route of the annual July 4 Tub Race from Mine Falls to Main Street. In March 1936, the river produced Nashua's most severe flood, with the water rising 12 feet, destroying $2,000,000 worth of buildings, bridges, and houses and leaving 2,000 people homeless; a high water mark on the Johns Manville Company doorway on East Hollis Street showed the flood's extent. In the 1830s, the Jackson Co. built a dam east of the Main Street Bridge to power its mills, then rebuilt it in 1847 for $8,000 and again in 1878 for $20,000, using wood and a temporary cofferdam during construction, while a fishway was added in 1878 to let fish bypass the dam. In 1907, the Jackson Co. built a concrete dam in front of the wooden dam, placing foundations in a pool that had once been a popular swimming hole and where boys were still diving when construction began. By the 1920s, the dam had been converted to hydropower and a power house had been built. After the mills closed, electricity generation stopped, then resumed in 1984 after the City of Nashua acquired the dam. Since 1977, the power house has housed several restaurants, first the Chart House and, beginning in 2000, Margaritas. When engineers held back the water to inspect the dam, the wooden skeletons of the earlier dams were rediscovered in the riverbed.