Complex sentence: The tunnel is over 18 miles (29 kilometres) in length, and spans through Greene County and Ulster County, and water is forced through the tunnel by gravity; it slowly declines at a rate of 4.4 feet per mile.
Split into simpler sentences: The tunnel is over 18 miles (29 kilometres) in length, and carries water through Greene County and Ulster County. Water is forced through the tunnel by gravity; it slowly declines at a rate of 4.4 feet per mile.