Changes in Streamflow Timing in the Western United States in Recent Decades Fact Sheet March 2005

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200 CD -a Cc oc-, 150 U U W 0 M a ca. co a 100 a� CM cc 50 aC--) June 19 June 9 May 30 May 21 May 10 April 30 Feb Mar Apr May Jun Jul Aug Sep O O Op O Qp v O b) Q O Cj0 ©O0 �`AO 00 O "00 O O O CIO 0 0 0 00 0 0 OO 0 0 O O 0 O 1920 1940 1960 1980 2000 Water Year (October - September) Figure 3. (a) Comparison of mean daily streamflows in the Clark Fork Yellowstone River, Wyoming, during the 1950s and 1990s, and (b) center -of- volume dates for each year from 1922 to 2003, with the red curve showing the 9 -year moving average and straight lines showing the trend before and after 1950. mean measured flows in the Clark Fork Yellowstone River, Wyoming, during the 1950s with those during the 1990s (fig. 3a). The overall river discharges in these two decades were quite similar, with average flow rates of 27.8 cubic meters per second in the 1950s and 27.9 in the 1990s. In the 1990s, however, springtime flows were larger and late summer flows were smaller than in the 1950s. Thus, flow generally arrived earlier in the 1990s than in 1950s, with an average center - of- volume date that was about 4 days earlier in the 1990s than in the 1950s. The year -by -year history of center- of -vol- ume dates for the Clark Fork Yellowstone River is shown in figure 3b, illustrating the general trend towards earlier flows in the river before, and especially after about 1950. Geographic Extent The geographic extent of the trends toward earlier streamflow in snow -fed streams is shown in figure 4, with tim- ing measured by the dates on which the snowmelt runoff adds a large initial surge to the rivers (fig. 4a) across the western conterminous states and by the center -of- volume dates in rivers throughout western North America (fig. 4b). Both measures indicate that flows in many western streams arrive a week to almost 3 weeks earlier now than they did in the middle of the 20th century. The largest changes have been identified in the Pacific North- west, but the trends also are present in the Sierra Nevada of California, in the Rocky Mountains, and through parts of British Columbia and southern Alaska. The regional trend in streamflow tim- ing developed in the midst of large year - to -year and basin -to -basin variations in both streamflow amounts and timing. The variations arc due to contrasts in topog- raphies, precipitation patterns, and snow conditions among the river basins. Trends toward later streamflow were detected in a few western rivers. Despite the basin - to -basin and year -to -year differences, however, over 90 percent of the stations with statistically significant trends in figure 4 have trended toward earlier run- off, signaling the breadth and strength of the regional trend in western states. The average center -of- volume date for rivers across the western United States is about 9 days earlier now than in the 1950s. Cause of Trends These trends in timing are most read- ily attributed to winter and spring warm- ing, as depicted in figure 2, but that inter- pretation is complicated by recent trends toward more (or less) precipitation in some areas and by a broad trend towards slightly later precipitation. The causes of these long -term climatic trends remain uncertain. The observed streamflow timing and winter - spring warming trends are consistent with current projections of how greenhouse effects may influence western climates and hydrology; thus streamflow timing and trends may be attributed, in part, to global warming. The climate of the North Pacific Ocean basin, however, underwent a seemingly natural change toward warmer conditions in the eastern Pacific and western Americas around 1977 (Mantua, and others, 1997). This multidecadal change was part of a long -term cycle of climate fluctuations in the region and to an uncertain extent, has contributed to long -term climatic and hydrologic changes in the western states during the past 50 years. Continued and even enhanced streamflow monitoring and analysis of western snow -fed rivers will be needed to determine the precise natural and human - induced causes, and the likely future, of these western streamflow- timing trends. If the trends have natural origins, then they may well reverse themselves, but if they are driven by man -made influences on the climate system, the streamflow timing may continue to change. If the trends continue their present course, then the "natural res- ervoirs" provided by western snowfields will become progressively less useful for water- resources management, flood risks may change in unpredictable ways, and many mountain landscapes will endure increasingly severe summer - drought conditions. I1 a) Water Years f — 1950- 1959 1999 Feb Mar Apr May Jun Jul Aug Sep O O Op O Qp v O b) Q O Cj0 ©O0 �`AO 00 O "00 O O O CIO 0 0 0 00 0 0 OO 0 0 O O 0 O 1920 1940 1960 1980 2000 Water Year (October - September) Figure 3. (a) Comparison of mean daily streamflows in the Clark Fork Yellowstone River, Wyoming, during the 1950s and 1990s, and (b) center -of- volume dates for each year from 1922 to 2003, with the red curve showing the 9 -year moving average and straight lines showing the trend before and after 1950. mean measured flows in the Clark Fork Yellowstone River, Wyoming, during the 1950s with those during the 1990s (fig. 3a). The overall river discharges in these two decades were quite similar, with average flow rates of 27.8 cubic meters per second in the 1950s and 27.9 in the 1990s. In the 1990s, however, springtime flows were larger and late summer flows were smaller than in the 1950s. Thus, flow generally arrived earlier in the 1990s than in 1950s, with an average center - of- volume date that was about 4 days earlier in the 1990s than in the 1950s. The year -by -year history of center- of -vol- ume dates for the Clark Fork Yellowstone River is shown in figure 3b, illustrating the general trend towards earlier flows in the river before, and especially after about 1950. Geographic Extent The geographic extent of the trends toward earlier streamflow in snow -fed streams is shown in figure 4, with tim- ing measured by the dates on which the snowmelt runoff adds a large initial surge to the rivers (fig. 4a) across the western conterminous states and by the center -of- volume dates in rivers throughout western North America (fig. 4b). Both measures indicate that flows in many western streams arrive a week to almost 3 weeks earlier now than they did in the middle of the 20th century. The largest changes have been identified in the Pacific North- west, but the trends also are present in the Sierra Nevada of California, in the Rocky Mountains, and through parts of British Columbia and southern Alaska. The regional trend in streamflow tim- ing developed in the midst of large year - to -year and basin -to -basin variations in both streamflow amounts and timing. The variations arc due to contrasts in topog- raphies, precipitation patterns, and snow conditions among the river basins. Trends toward later streamflow were detected in a few western rivers. Despite the basin - to -basin and year -to -year differences, however, over 90 percent of the stations with statistically significant trends in figure 4 have trended toward earlier run- off, signaling the breadth and strength of the regional trend in western states. The average center -of- volume date for rivers across the western United States is about 9 days earlier now than in the 1950s. Cause of Trends These trends in timing are most read- ily attributed to winter and spring warm- ing, as depicted in figure 2, but that inter- pretation is complicated by recent trends toward more (or less) precipitation in some areas and by a broad trend towards slightly later precipitation. The causes of these long -term climatic trends remain uncertain. The observed streamflow timing and winter - spring warming trends are consistent with current projections of how greenhouse effects may influence western climates and hydrology; thus streamflow timing and trends may be attributed, in part, to global warming. The climate of the North Pacific Ocean basin, however, underwent a seemingly natural change toward warmer conditions in the eastern Pacific and western Americas around 1977 (Mantua, and others, 1997). This multidecadal change was part of a long -term cycle of climate fluctuations in the region and to an uncertain extent, has contributed to long -term climatic and hydrologic changes in the western states during the past 50 years. Continued and even enhanced streamflow monitoring and analysis of western snow -fed rivers will be needed to determine the precise natural and human - induced causes, and the likely future, of these western streamflow- timing trends. If the trends have natural origins, then they may well reverse themselves, but if they are driven by man -made influences on the climate system, the streamflow timing may continue to change. If the trends continue their present course, then the "natural res- ervoirs" provided by western snowfields will become progressively less useful for water- resources management, flood risks may change in unpredictable ways, and many mountain landscapes will endure increasingly severe summer - drought conditions.