NEWBURYPORT, Mass. - There's a curious photo in the city's historical archives that captures a moment literally frozen in time.
It's the winter of 1897, and nine men stand on the Merrimack River, a few dozen yards offshore from what is now Newburyport's downtown waterfront boardwalk. The photographer is much further out on the ice - perhaps 200 feet or so.
The photo shows something that was once a fairly frequent natural phenomenon, but hasn't happened in decades. The Merrimack River used to freeze so solidly that horses and sleighs could safely cross it from Newburyport to Salisbury. People would walk from one side to the other, over a thick sheet of solid ice.
No one would dare try such a thing today. It's impossible. Even in bitterly cold winters there's always open water near downtown Newburyport, and the ice on the river's edge is thin and dangerous to walk on. Our lower end of the river no longer freezes all the way across.
It's a question that the experts can't answer definitively. There are a number of possible and interesting culprits, nearly all of which reflect the incremental changes that mankind has made on the local environment. Add them all together, and it's clear that this is a very different river than the one that Newburyport's citizens knew a century ago.
"It's hard to pinpoint why the river isn't freezing anymore," said Roy Sokolow, a hydrologist for the United States Geological Service's Massachusetts Water Science Center. "You can make some assumptions, but I don't like to make assumptions without data."
This year there's an anomaly that has made it impossible for the river to freeze across. The work on the Interstate 95 bridge project has brought with it icebreaking boats that are cutting through the ice on a daily basis. But that anomaly aside, observers say its clear that the overall pattern is less ice in the Merrimack, particularly at this lower end.
Incidents of the Merrimack River freezing all the way across are not documented in a formal manner. Instead, the information is provided through anecdotes - newspaper clippings, old photographs, diary entries and history books.
Joe Callahan, a Salisbury resident, amateur historian and retired Newburyport Fire Department employee, has compiled several of these anecdotes. They date back to 1785, when the river ice was reported to be thick enough to walk across as late as April 13 of that year. A similar report was made in April 1843, when a sleigh with two horses and several people crossed over the river.
Other anecdotes of river crossings include cold Januaries in 1857, 1859, 1867 and February of 1881. The year 1897 was notable in part due to the photo of icewalkers described in this story.
The year 1918 was perhaps one of the most noteworthy years. As Callahan's research showed, in February the ice was a foot thick in the river channel - a broad area in the center of the river. It was reported to be the thickest ice since the winter of 1867. The temperature was 20 below zero in Newburyport on Feb. 4. Trains were not running and cars were stalled out all over the region.
Callahan said he has heard reports of the river freezing thickly into the 1920s, and he recalls looking out from the Newburyport Fire Station and seeing it frozen across as late as the 1950s. But then something changed.
That change, according to experts, had been quietly at work for years. While Sokolow noted its not possible to pinpoint a single cause, there are a number of known variables.
Different water levels
One of them is an increase in the amount of water flowing down the river in the wintertime. That change is caused primarily by dams that have increased what was once a low flow of water in the winter months. Low flow tends to allow water to freeze more readily.
While some of the dams have been in existence for more than 150 years, some significant changes occurred during the late 1930s through the 1950s when the Army Corps of Engineers built a series of flood control reservoirs. Those controls are meant to regulate the flow of water and keep the river above low flow levels, according to David Vallee, the hydrologist-in-charge for NOAA's Northeast River Forecast Center.
There's also been significant changes in the atmosphere.
An increase in urbanization along the river also caused significant changes. More development means more rapid runoff during storm events, Sokolow said. That means more water in the river, and a faster flow.
Another key factor is climate change. While this has been an unusually cold winter, Vallee noted that winters are not as severe as they once were. The last gasp of extraordinarily cold winters occurred periodically through the 1920s to 1960s, he noted. Average air and water temperatures have risen.
So have sea levels, which increases water flow.
What's in the river?
One of the unknown factors is the impact of contaminants in the river. It was only fairly recently - within the past couple of decades or so - that the government began conducting regular and comprehensive water quality studies. About every three years or so the tests are done.
Prior to that, Sokolow said tests were done sporadically, not frequently enough to get a good stream of data together to show how contaminants were changing the river.
"Unfortunately they weren't doing water quality tests way back when," he said.
Another unknown factor is "conductiveness" - the impact of salt on river freezing. The saltier the water, the harder it is for it to freeze. Fresh water freezes at 32 degrees Fahrenheit; salt water at 28 degrees.
While there seems anecdotally to be more fresh water in the river now than there was many years ago, it is difficult for scientists to say whether changes in salinity has affected the river. Indeed, Sokolow said a water monitor located about eight miles upstream from the Route 1 Gillis bridge indicates that the river is saltiest during the summertime.
Sokolow said it would be interesting to know what the precise causes are for the freezing patterns. But the Merrimack is hardly alone in posing conundrums for scientists.
"There's a lot of mysteries in the rivers around the state," he said.