The annual timing of river flows is a good indicator of climate-related changes, or lack of changes, for rivers with long-term data that drain unregulated basins with stable land use. Changes in the timing of annual winter/spring (January 1 to May 31) and fall (October 1 to December 31) center of volume dates were analyzed for 27 rural, unregulated river gaging stations in New England, USA with an average of 68 years of record. The center of volume date is the date by which half of the total volume of water for a given period of time flows past a river gaging station, and is a measure of the timing of the bulk of flow within the time period. Winter/spring center of volume (WSCV) dates have become significantly earlier ðp , 0:1Þ at all 11 river gaging stations in areas of New England where snowmelt runoff has the most effect on spring river flows. Most of this change has occurred in the last 30 years with dates advancing by 1 -2 weeks. WSCV dates were correlated with March through April air temperatures ðr ¼ 20:72Þ and with January precipitation ðr ¼ 20:37Þ: Three of 16 river gaging stations in the remainder of New England had significantly earlier WSCV dates. Four out of 27 river gaging stations had significantly earlier fall center of volume dates in New England. Changes in the timing of winter/spring and fall peak flow dates were consistent with the changes in the respective center of volume dates, given the greater variability in the peak flow dates. Changes in the WSCV dates over the last 30 years are consistent with previous studies of New England last-frost dates, lilac bloom dates, lake ice-out dates, and spring air temperatures. This suggests that these New England spring geophysical and biological changes all were caused by a common mechanism, temperature increases. Published by Elsevier Science B.V.

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Changes in snowmelt-related streamflow timing have implications for water availability and use as well as ecologically relevant shifts in streamflow. Historical trends in snowmelt-related streamflow timing (winter-spring center volume date, WSCVD) were computed for minimally disturbed river basins in the conterminous United States. WSCVD was computed by summing daily streamflow for a seasonal window then calculating the day that half of the seasonal volume had flowed past the gage. We used basins where at least 30 percent of annual precipitation was received as snow, and streamflow data were restricted to regionally based winter-spring periods to focus the analyses on snowmelt-related streamflow. Trends over time in WSCVD at gages in the eastern U.S. were relatively homogenous in magnitude and direction and statistically significant; median WSCVD was earlier by 8.2 days (1.1 days/decade) and 8.6 days (1.6 days/decade) for 1940-2014 and 1960-2014 periods respectively. Fewer trends in the West were significant though most trends indicated earlier WSCVD over time. Trends at low-to-mid elevation (< 1,600 m) basins in the West, predominantly located in the Northwest, had median earlier WSCVD by 6.8 days 0.9 days/decade) and 3.4 days (1960-2014, 0.6 days/decade). Streamflow timing at high-elevation (≥ 1,600 m) basins in the West had median earlier WSCVD by 4.0 days (1940-2014, 0.5 days/decade) and 5.2 days (1960-2014, 0.9 days/decade). Trends toward earlier WSCVD in the Northwest were not statistically significant, differing from previous studies that observed many large and (or) significant trends in this region. Much of this difference is likely due to the sensitivity of trend tests to the time period being tested, as well as differences in the streamflow timing metrics used among the studies.Mean February-May air temperature was significantly correlated with WSCVD at 100 percent of the study gages (field significant, p < 0.0001), demonstrating the sensitivity of WSCVD to air

River base flow is important to aquatic ecosystems, particularly because of its influence on summer water temperatures. Summer (June through September) daily mean streamflows were separated into base flow and stormflow components by use of an automated method at 25 stations in the New England region of the United States that drain predominantly natural basins. Summer monthly mean base flows increased from 1950–2006 at most stations in western New England with many large increases (>20%) and some very large increases (>50%) in and near New Hampshire and Vermont. The same was true for increases in summer 7 day low base flows in and near New Hampshire and Vermont during this same period; in contrast, there were small and large decreases in 7 day low base flows in northern and coastal areas of Maine. Summer stormflows increased from 1950–2006 by more than 50% at many stations in New England, particularly in and near New Hampshire and Vermont. The increases in base flows and stormflows at many stations in and near New Hampshire and Vermont were likely driven by the large increases in summer precipitation recorded at weather stations in this area. Summer rainfall increased at most weather stations in New England from 1950–2006 with many increases of more than 20% in western New England. Summer air temperature increased on average by 1.1°C from 1950–2006 in New England and may have played a role in the decreased base flows in northern and coastal Maine through increased evapotranspiration. Many variables increased less from 1930–2006 than from 1950–2006.