Cayuga inhabited the river valley and lakeshores in between. All three were part of the Iroquois League, which is believed to have been established between 1570 and 1600. For hundreds of years before the arrival of Europeans, the river was an important Native American trade route. Explorers and settlers The first Europeans to reach the Seneca River were likely Jesuit missionaries in the late 1600s, who established an outpost, St. Stephen, on the river shortly below its origin at Seneca Lake. Canalization In 1821 the Seneca Lock Navigation Company completed eight locks along the upper Seneca River above Cayuga Lake to allow navigation to Seneca Lake. By 1828 this had been replaced by a state-owned waterway, the Cayuga-Seneca Canal, which contains eleven locks in . Construction of the Seneca reach of the Erie Canal began in the 1820s. The channel between Three Rivers and Cayuga Lake was widened and straightened to accommodate barges, and other reaches were bypassed via the construction of parallel canals. The canal path had to cross the Seneca River at several points, so locks were built to lower boats down to river level, where they were towed across aided by temporary wooden bridges. In 1849 work began to separate the canal from the river, in order to reduce the impact of flooding and sedimentation. The Montezuma Marshes at the outlet of Cayuga Lake were a major obstacle to the Erie Canal path. The stone Seneca River Aqueduct (Richmond Aqueduct), which carried the canal over the Seneca and Clyde Rivers, opened in 1857 after eight years of construction. At it was the second-longest aqueduct on the Erie Canal system. Most of the aqueduct was dynamited in the 1910s to allow navigation on the Barge Canal. Certain points on the Seneca River was an early center of development for industry. Seneca Falls is the location of the only significant natural drop on the river, which was utilized in the early days to power water mills. Where the river had no natural falls, mill dams were built, one of the earliest of which was at Baldwinsville. In 1915 a dam high was built at Seneca Falls to generate hydroelectricity. Ecology and environmental issues Below Onondaga Lake, the Seneca River is moderately polluted by industrial and domestic waste, including high levels of mercury, PCBs, dioxin and ammonia. The New York State Department of Health advises limited consumption of fish from