Section 4. Average Daily Flow

Streamflow patterns in Puget Sound rivers and streams are classified into three hydrologic regimes: snowmelt dominated, rainfall dominated, and transitional (Stewart et al. 2005, Beechie et al. 2006, Elsner et al. 2009). Generally, in snowmelt-dominated rivers, a significant proportion of winter precipitation is stored as snowpack, resulting in low winter flows with peak flows during the spring snowmelt period from April through July. Rainfall-dominated rivers experience peak flow during the winter months as the majority of precipitation falls as rain. Transitional rivers experience both winter and spring peak flows resulting from winter precipitation and spring snowmelt. Hydrologic flow regimes in Puget Sound rivers have been altered through the construction of dams for flood control or power generation, or by changes in land cover and climate. Alteration of historical flow patterns can cause ecological harm and disrupt supply (Poff et al. 1997, Wiley and Palmer 2008).

Barnett et al. (2008) utilized a multivariate analysis to evaluate simultaneous changes in average winter temperature, snow pack, and runoff timing in the Western United States (including the Washington Cascades) for the period from 1950 – 1999. They found significant increasing trends in winter temperature and decreasing trends in snow pack and runoff timing (indicating earlier snowmelt). In order to distinguish natural variation from anthropogenic forcing, they evaluated the observations against two separate climate models and found that the hydrologic changes were both detectable and attributable to anthropogenic forcings.

Stewart et al. (2004) investigated historic (1948-2000) and future streamflow timing in snowmelt dominated rivers and streams in the Western United States. They found significant trends towards earlier runoff in many rivers and streams in the Pacific Northwest. Utilizing a ‘business-as-usual’ emissions scenario with a Parallel Climate Model, they predicted continuation of this trend, due largely to increased winter and spring temperatures but not changes in precipitation. In a companion study they further analyzed the trends in streamflow timing with variations of the PDO (Stewart et al. 2005). While streamflow timing was partially controlled by the PDO there remained a substantial portion of the variation in timing that was explained by a longer-term warming trend in spring temperatures.

The Climate Impact Group at the University of Washington performed The Washington Climate Change Impact Assessment. The assessment included analyses of hydrology and water resource management utilizing results from 20 global climate models and two emissions scenarios from the IPCC Special Report on Emissions Scenarios (A1B and B1) to evaluate projected changes in spring snowpack and runoff (Elsner et al. 2009). For the rivers in the Puget Sound basin, they found a dramatic decrease in spring snowpack with there being almost no April 1 snowpack by 2080. Change in snowpack was correlated with a predicted change in river hydrography, from transition- or snow-rain dominated, to rain dominated patterns. There was little predicted change in annual precipitation.

River hydrographs showing average annual daily flow from the initiative of observation through 1968, and from 1984 through 2009 are presented in Figure 1. Much of the warming trend observed in the Pacific Northwest has occurred since 1975 (Hamlet and Lettenmaier 2007). Comparing the streamflow patterns before and after this period could indicate effects of climate change.

Figure 1. Average daily flow shown for historic (pre 1970’s) and recent (mid-1980s to present) time period for 14 Puget Sound rivers. The time period varies slightly between river systems based on availability of data. Colored lines show average daily flow averaged over time period indicated in each of the chart title. Dark lines are 14-day smoothed averages for historic (dashed) or recent (solid) time periods. Data taken from United States Geological Service.

There is considerable variation even in averaged data which makes the detection of long-term trends problematic. However, the following generalities emerge. First, there has been little change in hydrologic patterns in rainfall-dominated rivers (Samish, Stillaguamish, and Deschutes) or in the snowmelt-dominated river (Thunder Creek). It is possible that consistent glacier melt contributed to the stable patterns in the latter river. However, there was an observable decline in spring peak flows in all of the transitional rivers (Nooksack, Sauk, Newhalem Creek, Skykomish, Upper Cedar, Upper Puyallup, Upper Nisqually, and Duckabush). Moreover, there appears to be a decline in the magnitude of the summer 7-day average low flows.

The analysis presented above was derived from data in the public domain. Hydrographs were created utilizing average daily discharge data from USGS stations located in the Puget Sound region (United States Geological Survey 2010b). The datasets include qualification codes indicating whether data are provisional or have been approved (United States Geological Survey 2010a). We avoided using provisional data in this analysis, and we omitted data from gauging stations for which advisory notes warning against unreliable data quality had been posted.

The analysis in this section is qualitative and intended to illustrate potential changes in streamflow patters over time. Consequently, statistical significance was not determined. Specific streamflow measures, such as annual 7-day average low flow, or centroid of flow timing, are quantitative measures that can be evaluated statistically and are presented elsewhere in this document.

There is some evidence for changes in transitional river systems over time, indicated primarily as decreasing magnitude of the spring snowmelt peak flows. This is consistent with published predictions for the western North America. There also appears to be a decrease in the magnitude of summer low flows in transitional river systems. There was less evidence for change in daily flow patterns for rainfall-dominated or snowmelt-dominated river systems. Because of variation in hydrologic alteration, particularly between rivers or streams of differing classifications, combining streamflow information across multiple streams to evaluate general status and trends may not be appropriate and results should be interpreted with caution.

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Beechie, T., E. Buhle, M. Ruckelshaus, A. Fullerton, and L. Holsinger. 2006. Hydrologic regime and the conservation of salmon life history diversity. Biological Conservation 130:560-572.

Elsner, M. M., L. Cuo, N. Voisin, J. S. Deems, A. F. Hamlet, J. A. Vano, K. E. Mickelson, S.-Y. Lee, and D. P. Lettenmaier. 2009. Implications of 21st Century Climate Change for the Hydrology of Washington State. JISAO Climate Impacts Group, University of Washington, Seattle, WA.

Hamlet, A. F. and D. P. Lettenmaier. 2007. Effects of 20th century warming and climate variability on flood risk in the western U.S. Water Resources Research 43.

Poff, N. L., J. D. Allan, M. B. Bain, J. R. Karr, K. L. Prestegaard, B. D. Richter, R. E. Sparks, and J. C. Stromberg. 1997. The natural flow regime. Bioscience 47:769-784.

Stewart, I. T., D. R. Cayan, and M. D. Dettinger. 2004. Changes in snowmelt runoff timing in western North America under a 'business as usual' climate change scenario. Climatic Change 62:217-232.

Stewart, I. T., D. R. Cayan, and M. D. Dettinger. 2005. Changes toward earlier streamflow timing across western North America. Journal of Climate 18:1136-1155.

United States Geological Survey. 2010a. Provisional data for Washington.

United States Geological Survey. 2010b. USGS Washington Water Science Center.

Wiley, M. W. and R. N. Palmer. 2008. Estimating the impacts and uncertainty of climate change on a municipal water supply system. Journal of Water Resources Planning and Management-Asce 134:239-246.