Trends in Streamflow, River Ice, and Snowpack for Coastal River Basins in Maine During the 20th Century

Trends over the 20th Century were examined in streamflow, river ice, and snowpack for coastal river basins in Maine. Trends over time were tested in the timing and magnitude of seasonal river flows, the occurrence and duration of river ice, and changes in snowpack depth, equivalent water content, and density. Significant trends toward earlier spring peak flow and earlier center-of-volume runoff dates were found in the extended streamflow record spanning 1906-21 and 1929-2000. Only one of the six coastal rivers in the study analyzed for trends in cumulative runoff had a significant change in total annual runoff volume. Last spring river-ice-off dates at most coastal streamflow-gaging stations examined are trending to earlier dates. Trends in later fall initial onset of ice also are evident, although these trends are significant at fewer stations than that observed for ice-off dates. Later ice-on dates in the fall and (or) earlier ice-off dates in the spring contribute to a statistically significant decrease over time in the total number of days of ice occurrence at most gaging stations on coastal rivers in Maine. The longest, most complete snow records in coastal Maine indicate an increase in snow density for the March 1 snow-survey date during the last 60 years. The historical
trends in streamflow, ice, and snow are all consistent with an earlier onset of hydrologic spring conditions in coastal Maine.