By Daniel Adams
The Charles River Basin — at least, the one we recognize — was born on Oct. 20, 1908.
Before then, much of the waterway between Boston and Cambridge was an estuary, with expansive, foul-smelling mud flats emerging at low tide. But on that Tuesday more than a century ago, workers on the first dam at the mouth of the Charles lowered 84 massive timber gates into the water, creating the placid, lake-like river so familiar today.
Now, for the first time since it was submerged, the riverbed is coming back into view, this time in vivid digital detail. A Massachusetts Institute of Technology engineer and a longtime sailor and scientist have collaborated to create the most thorough, accurate chart of the lower Charles yet. It promises to become a foundational tool of multiple efforts to study, navigate, and manage the increasingly crowded Charles.
“The Charles is now one of the busiest recreational rivers in the country, but I can tell you from personal experience: The data that’s available out there is just not right,” said Carl Zimba of the Charles River Alliance of Boaters, or CRAB, a scientist who has sailed on the river for decades. “The focus of our project was to generate hard data on the real depth of the river.”
The lack of such data was surprising to many. After all, the Charles is teeming with boaters and located between several large cities — hardly remote or unexplored territory.
But the few existing charts of the river were approximate at best, Zimba said, cobbled together from dubious or obsolete data. So while ships in commercially navigable waters travel in channels marked with a standardized buoy system, recreational boaters on the Charles instead rely on experience, landmarks, and a complex “honor system” negotiated by various clubs and universities to avoid collisions and shallows.
The charting effort began in 2015, when a friend at the Head of the Charles Regatta asked Zimba for a chart to help place booms and buoys along the race course. Finding none of usable quality, he contacted a friend, Michael Sacarny, a research engineer at MIT’s Sea Grant College developing technology for autonomous boats. Could one of the university’s robot boats map the Charles?
Sacarny was game, and he commandeered MIT’s 16-foot autonomous catamaran Rex. To the boat’s suite of sophisticated cameras, lasers, and other sensors, he added a relatively ordinary gadget: a sonar fish-finder of the kind found in most outdoor-equipment stores.
With Sacarny and Zimba in a chase boat, Rex swept a portion of the Charles by MIT in a neat, lawnmower-like grid, pinging the bottom with sonar pulses from the fish-finder. The sonar data and GPS coordinates were loaded into map-making software that wove the raw numbers into a high-resolution chart.
“We intentionally used proven, off-the-shelf technology,” Zimba said, “because it allows you to go back in five years, measure the same areas again, and know that you’re comparing apples to apples.”
Encouraged, Sacarny and Zimba drummed up money to fund a full-scale survey of the 9.5-mile section of the lower Charles to the Watertown dam.
As they moved into the shallower, narrower waters upriver, however, the team had to dock Rex in favor of a conventional motor boat. The autonomous ship struggled to navigate tight quarters, and its safety system killed the engines whenever rowers and other boaters got too close — which was often.
Over the following months, the two took numerous trips, squeezing the simple sonar-and-GPS rig into every cove along the way. They had to adjust depth readings to account for changes in the water level because of rain or releases by the dam.
The chart, now nearly finished, hints at the Charles’s past: Trenches of deeper water trace the narrower river channel that existed before the 1908 damming. The mud flats that once surrounded that channel, meanwhile, show up as large plateaus of uniform shallowness. And underneath the Harvard Bridge, the chart reveals a channel where a swing span once rotated to allow larger ships to pass; it was replaced with a fixed span in 1924.
The map also spotlights areas of potential concern, including a shallow shelf of sediment near the Watertown Yacht Club. Another mound, where the Muddy River joins the Charles near Kenmore Square, is just 18 inches under the surface, a hazard to all but the smallest boats. Follow-up studies will measure how quickly the sandy muck is piling up.
“You can get out and stand on it,” Zimba said. “It’s a problem.”
A second “side-scan” sonar unit recorded fuzzy images of objects under the water, including tires, pilings of wrecked piers, and a tangle of bent pipes, probably the remnants of a defunct “bubbler” system the state installed decades ago in an attempt to oxygenate the fetid waters.
Peering at the hidden underwater world is “almost magical,” Sacarny said. “There are undoubtedly stories that go along with all these things.”
The team plans to publish its chart in several formats, including as a printed booklet, an interactive digital chart, and a format for Google Maps and Google Earth.
Before the MIT-CRAB project, the most recent chart was published in 2000 by the United States Geological Survey as part of a pollution study. However, it is outdated and imprecise: Researchers videotaped the screen of a fish-finder and transcribed the depths by hand later. The 2000 version gives boaters only a rough picture of the bottom and is a static image, not an interactive chart.
Yet with the cleaned-up waters of the Charles drawing ever-larger numbers of boaters and tourists, river groups and state agencies badly need a good picture of the Charles’s depths.
Last winter, for example, the state spent $800,000 dredging an accumulating sandbar near Watertown where boats were running aground. And with the Charles River Conservancy planning a swimming area near the Museum of Science, it will be crucial to understand how and where polluted muck at the bottom of the river moves over time.
“Once we have high-quality information everyone can get behind, instead of just anecdotes, then you can have a more rational discussion about, ‘Now what do we do?’ ” Zimba said.
Conservancy president Renata von Tscharner said the new chart will make boating safer and boost efforts to study and use the river.
The new mapping data might also be useful to Northeastern University professor Ferdi Hellweger, who’s working on a digital simulation of the river’s flow. His software could be used to determine when swimming areas should close by predicting how bacteria and other pollutants travel through the Charles after heavy rain.
Boston Duck Tours, whose restored World War II-era amphibious vehicles take tourists on tours of the Charles, helped fund the charting project. Jim Healy, the company’s safety officer, said the lack of a reliable chart forced him to take his own depth readings several years ago to check the routes his boats took and to find locations where a duck boat could exit the river in an emergency.
“The soundings I took couldn’t possibly be as detailed,” Healy said.
“The river’s never been charted to the extent that they’ve done it. It’s rather impressive.”