Changes in the number and timing of days of ice-affected flow on northern New England rivers, 1930-2000

Historical dates of ice-affected flows for 16 rural, unregulated rivers in northern New England, USA were analyzed. The total annual days of ice-affected flow decreased significantly (p < 0.1) over the 20th century at 12 of the 16 rivers. On average, for the nine longest-record rivers, the total annual days of ice-affected flow decreased by 20 days from 1936 to 2000, with most of the decrease occurring from the 1960s to 2000. Four of the 16 rivers had significantly later first dates of ice-affected flow in the fall. Twelve of the 16 rivers had significantly earlier last dates of ice-affected flow in the spring. On average, the last dates became earlier by 11 days from 1936 to 2000 with most of the change occurring from the 1960s to 2000. The total annual days of ice-affected flow were significantly correlated with November through April air temperatures (r = -0.70) and with November through April precipitation (r = -0.52). The last spring dates were significantly correlated with March through April air temperatures (r = -0.73) and with January through April precipitation (r = -0.37). March mean river flows increased significantly at 13 of the 16 rivers in this study.