PORTLAND – A significant decline in the number of days in which ice affected the flow of northern New England rivers provides additional evidence that the region’s winters are not as harsh as they used to be, a new study has found.

The U.S. Geological Survey study, published in the journal Climatic Change, was based on data from gauging stations in Maine, New Hampshire and Vermont that measure the height and flow of 16 rivers.

Researchers found that 12 of the rivers showed a significant decrease during the 20th century in the number of days each year when there was enough ice to break down what is normally a consistent relationship between the river’s height and flow.

The total winter days of ice-affected flow decreased by 20 days from 1936 to 2000 for the nine longest-record rivers, with most of the decreases occurring since the 1960s.

While only four of the 16 rivers had significantly later first dates of ice-affected flow in the fall, 12 had significantly earlier ice-out dates in the spring, the study found. The average ice-out date came 11 days earlier from 1936 to 2000, again with most of the change taking place since the 1960s.

“The changes in spring river ice-outs in northern New England are consistent with previous studies,” said Glenn Hodgkins, a USGS hydrologist in Augusta and lead author of the study. “The overall evidence of changes is strong and is consistent with warming temperatures in the late winter and spring in New England in the last 30 to 40 years.”

He noted that questions of “whether the warmer climate in New England is linked to global climate change are beyond the scope of these studies.”