During the summer months, visitors can experience one of the great marvels of American engineering: the Northern Canal. Finished in 1847, the Northern Canal runs along the Merrimack River separated by a 36 foot high granite wall later named the "Great River Wall."
Update- July 23, 2017: The walkway is open! Flow rates have finally come down low enough to open the walkway. Forecasts are around 3500 cubic feet per second in the next couple of days, so we expect the walkway to remain open for most of the week. Typically, flow rates are lower, and if that holds true this year, the walkway may be open for most of the rest of the summer.
The Northern Canal Walkway (which includes the Great River Wall) provides breathtaking views of the Merrimack River, the canal and the surrounding areas. However, access to the Northern Canal Walkway is only available when the flow rate in the Merrimack River is under 3500 cubic feet per second.
To find out if the Northern Canal Walkway is open, please visit the USGS flowrates page and check the "most recent instantaneous value" towards the bottom of the page. If this value is below 3500 the walkway will be open. (see screenshot of the page below)