The river, once a largeish harbor estuary that ran into Cambridge and Somerville, now starts at the Charles under the Zakim by the equally Big-Dig funded Paul Revere Park and ends roughly at the Bunker Hill Community College parking lot off Rutherford Avenue - and you can follow a path along much of that.
If you enter from the Rutherford Avenue side (the entrance is just south of the parking lot - you can't miss the Seussian light fixtures), one of the first things you come across is a statue of sacks of potatoes:
The statue harkens back to the days when what is now the college parking lot was filled with sheds into which workers offloaded Maine potatoes from trains from up north. That all ended with a big fire in 1962 - the area smelled like roasted potatoes for quite some time.
The walkway that goes by the potatoes is etched with numbers marking the one-time depth of the river at those points:
As you can see from the first photo, there's now actual vegetation along the banks of what used to be an incredibly polluted creek - one slaughterhouse just dumped its offal right in.
Although the river's cleaner nowadays, one thing that won't be coming back are the oysters that used to grow there. They require salt water, which is now blocked by the Charles River Dam.
That there is any green at all is kind of amazing given that the river is covered by highway ramps and the I-93 connection between the Zakim and the old Deck and surrounded by train tracks and a sand and gravel plant.
A swoopy new pedestrian bridge links Paul Revere Park to parks on the Cambridge side of the Charles:
At its, um, headwaters, still waters don't run deep:
One of the first things the state did when it started down the road of replacing the Central Artery was coming up with an official name and logo for the project, which today you see only on the odd manhole cover:
Getting there: Take the Orange Line to Community College, then walk towards downtown Boston on Rutherford Avenue. Or walk through Paul Revere Park under the Zakim and look for the path.
At the other end of the Big Dig:
The end of Fort Point Channel.