In Boston, behind North Station/New Boston Garden Arena, is the Charles River Dam and Locks, which marks the boundary where the Charles River meets Boston Harbor, which, in turn, merges into the Atlantic Ocean.
The Charles River New Dam and Locks (we'll get to the old dam and locks, later), is accessible either from the Charlestown or the West End sides of the river. From the West End side (the core of Boston, starting at Causeway Street, walk to the east side of the Boston Garden Arena and the I-93 lanes down a paved way that leads to a parking area, then pass by the State Police Barracks on the right. The locks are ahead. From Charlestown, go to Paul Revere Park and walk along the path down to a grassy area, then walk toward the river to the locks. There is a walkway that traverses the locks - read the signs for your safety.
The river passes through the locks when its not high tide on the other side. If there is high river water due to storms, there are a bank of pumps that can force the water out to the harbor. But this rarely happens.
The Charles River is a beautiful and historic river that separates Boston and Cambridge. The new dam, built in 1977, helps form the Charles River Basin upstream, which is a wide lake that is lined with parkland on both sides. The most well known of these public spaces is the Esplanade, which features the Hatch Shell where the Boston Pops plays the July 4th concert.
Before 1977, the Charles River Old Lock and Dam provided that barrier. It still exists - the Museum of Science is situated on the barrier. However, the lock is always open - though, occasionally, the draw bridge over it opens to allow the occasional yacht or motor boat through. This lock was built in 1910. This event changed the mouth of the Charles profoundly.
Before the first lock and dam, the Charles was lined with salt marshes on both sides. As Boston grew, these marshes were filled in and built on. This left properties exposed to flooding, and the remaining marshes couldn't handle the sewage and runoff, resulting in a putrid mess. The dam prevented tidal effects and somewhat mitigated floods and outflows, but a series of hurricanes - in particular, Carol in 1954, showed how insufficient the old dam was, and, thus, the present dam and locks was built.
The Charles River isn't a large river. There are tributaries to the Connecticut and Merrimack Rivers that are larger than the Charles. However, it is a landmark of Boston and was the setting for Paul Revere's crossing near this location and is the inspiration for the song, "Love that Dirty Water." In the fall, the Charles River Regatta is held where colleges race one another in rowing competitions.
Now, about that 'dirty water.' In the past the Charles River was polluted, not only from the runoff mentioned above, but also from pollution upstream, largely from the industry that grew along the banks and may have even tapped the water in the past for power. Efforts by a non-profit group, Charles River Watershed Association, that include running two water quality monitoring programs, have greatly improved the water to the point where some areas are now swimmable.
The Charles River begins in Hopkinton, at Echo Lake, which is a public drinking source. The tiny stream meanders through the southwest suburbs like Milford, Franklin, and Medfield, then, as a small river through Natick, Needham, and Newton. Between Newton and Waltham, the river slowly meanders through the Lakes District then makes it to Watertown where it flows over the dam to the Charles River Basin. The length is about 80 miles, though, as the crow flies, the source is only 29 miles from the mouth.
Other Sources:
US Army Corps of Engineers (Charles River Dam Local Protection Project):
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visit link)
Charles River Watershed Association:
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visit link)