Section 5. Seven-Day Low Flow

The hydrologic regime of rivers and streams in the Puget Sound is characterized by peak flows during the winter as a result of heavy precipitation, or during the spring due to snowmelt runoff. Base flows during the summer are low, consisting mainly of groundwater discharge. Base flows can be affected by climate change, urbanization, or groundwater withdrawals. Summer base flow levels are important ecologically because they can define or limit the availability of habitats. Summer base flow levels are important to water resource managers because low flows often coincide with peak consumption.

Climate change is expected to alter river hydrology in the Puget Sound basin. Observed and predicted increases in winter temperatures could result in more precipitation falling as rain instead of snow, earlier snowmelt timing, earlier streamflow timing, and lower summer flows (Mote et al. 1999, Mote et al. 2003). Several studies have evaluated the impacts of climate change on spring snowpack in the Pacific Northwest, with the conclusion that decreasing spring snowpack may result in lower summer flows. Long-term decline in snowpack in the Pacific Northwest was found to correlate largely with increasing temperatures, but not precipitation (Mote 2003). Follow-on studies with a Variable Infiltration Capacity model were performed to discern long term trends in spring snowpack from temperature and precipitation variability (Hamlet et al. 2005, Mote et al. 2005). Results suggested that long-term downward trends in spring snowpack were associated with widespread warming. Trends in snowpack associated with precipitation were largely controlled by decadal oscillations. Multiple regression analysis indicated that climatic oscillations accounted for approximately 10-60% of the trends in spring snowpack, depending on the time series examined (Mote 2006), leading the authors to conclude that the primary factor driving declining snowpack in the Washington Cascades was rising temperatures. The long-term snowpack trends were unrelated to the variability caused by Pacific oscillations.

Casola et al. (2009) investigated the potential impacts of climate change on snowpack by combining future temperature predictions with the estimated temperature sensitivity of spring snowpack. Analysis of historic and projected temperature data indicated that snowpack reductions over the past 30 years ranged from 8%-16% while future temperature change would result in an 11%-21% reduction in spring snowpack by 2050.

Stewart et al. (2005) evaluated the monthly fractional flow in snowmelt-dominated river systems in the Western United States and found an increasing fraction of flow occurring in March, corresponding with a decreasing fraction in June. Changes in streamflow pattern were associated with long-term increases in spring and winter temperatures, which spanned the decadal-scale Pacific climate oscillations. Barnett et al. (2008) utilized a multivariate analysis to evaluate the 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) associated with anthropogenic forcings.

The Climate Impacts Group utilized 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 projected a dramatic decrease in spring snowpack with almost no April 1 snowpack by 2080. The climate change-related alterations in spring snowpack and streamflow timing are expected to result in lower summer flows.

Land use alterations can also result in lower summer flows. Urbanization and development are associated with an increase in impervious surface resulting in higher runoff fractions and lower infiltration (Burges et al. 1998). Reduced infiltration can lead to lower base flows, although this effect can be somewhat offset by a reduction in evapo-transpiration from the clearing of trees (Cuo et al. 2008). The construction of storm drain systems has been implicated as a primary factor in the reduction a base flows (Simmons and Reynolds 1982).

Cuo et al. (2009) utilized a Distributed Hydrology-Soil-Vegetation Model in order to determine the relative effects of land cover and temperature change on flow patterns in Puget Sound streams. They found that the relative importance of temperature and land cover differed between the upland and lowland basins. In the lowland basins land cover changes were more important and generally resulted in higher peak flows and lower summer flows, primarily due to increased runoff. Both land use change and climate effects were important in the upland basins. Climate change had the largest impact in the transitional zones and resulted in higher winter flows, earlier spring peak flows, and lower summer flows. A similar modeling study of a basin located in the Portland, OR metropolitan area, using a single climate change simulation combined with a ArcView Soil and Water Assessment Tool, predicted an increase in overall flow, but a decrease in summer baseflow, by 2040 (Franczyk and Chang 2009).

Monitoring trends and predicting potential future alterations in streamflow patterns is important for water resource managers to ensure sufficient supply to meet demand (Snover et al. 2003, Milly et al. 2008). In the Pacific Northwest, summer low flows define the crucial period of water use and availability, and define system yield capacity. Wiley and Palmer utilized a three-stage modeling approach to evaluate the impacts of climate change on the water supply system for Seattle metropolitan region (Wiley and Palmer 2008). They predicted a decline of 6% per decade in July-September reservoir inflows resulting in a loss of available water in the system of approximately 56,000 acre-feet by 2040. Climate-related changes may reduce overall system yield.

Vano et al. (2009) expanded the analysis to include the Everett and Tacoma water supply systems. They predicted decreased summer reservoir inflows and storage for all three systems. System reliability, however, remained relatively strong assuming current demand.

Summer low flows in streams and rivers may be ecologically important. A substantial body of literature describes the potential deleterious impacts of low summer flows on fish survival (see Crozier et al. 2008, Palmer et al. 2009 and references therein). Potential negative biological impacts of low summer flows include high water temperatures, stranding, low dissolved oxygen, crowding, and disease. Although the strength of salmon runs has been shown to be positively and significantly correlated to summer stream flow in Puget Sound rivers, the actual causative mechanism is unclear due to complicated and interrelated variations between flow, temperature, habitat, and other variables (Mathews and Olson 1980). Rand et al. (2006) evaluated the potential effects of reduced flow and increased water temperature on upriver migration of Pacific salmon in the Frasier River. Lower discharge volumes during the migration period increased survival by decreasing energy requirements of the migrating salmon (making it easier to swim upstream) leading to a stronger pre-spawn population. Higher water temperatures, however, have been shown to increase metabolic rates and increase energy requirements. Presumably, within some range, the energetic benefits of decreased flow will compensate for costs from higher temperatures, yielding no net effect.

Scheuerell et al. (2006) used summer stream temperatures, which are predicted to increase with decreased flow, as a negative factor in survival of Chinook salmon in an effort to model salmon survival according to changes in various environmental conditions. Battin et al. (2007) predicted that Chinook salmon spawner capacity was proportional to minimum discharge during the spawning period; reductions in flow would result in reductions in spawning capacity due to habitat limitations. Low flows are also important for juvenile Coho due to space and food limitations, while low flows may be associated with temperature limitations in other areas (Ebersole et al. 2009). Trout survival and growth have been shown to be negatively associated with low stream discharge (Harvey et al. 2006, Berger and Gresswell 2009).

There remains substantial uncertainty in the predicted changes, related not only to climate change, but also to biological response and potential for adaptation among various species, particularly salmonids (Crozier et al. 2008, Schindler et al. 2008). Biological responses are likely to vary according to the specific stream and basin.

Summer 7-day average low flow is the metric chosen to represent low stream flow conditions. It is widely used and not susceptible to temporary upstream flow changes than may affect one-day low flow calculations (Riggs 1985). Annual values for 7-day average low flow were calculated using gauge data from 14 different locations on unregulated rivers within the Puget Sound, in order to evaluate the status and trends of low flows within the region (Table 1). Data from seven rivers indicated a significantly decreasing trend in 7-day average low flow for the time period on record (p<0.05). Data from three other rivers indicated decreasing trends in 7-day average low flow, although with a slightly higher degree of statistical uncertainty (p<0.10). Four rivers showed no significant trends in annual 7-day average low flow. Notably, no river system showed significantly increasing trends in annual 7-day average low flow. The average change for the rivers with significant trends in annual 7-day average low flow was -4.4% per decade.

Table 1. Average 7-day Low Flow for the time period of record, the annual rate of change of 7-day low flow, and the probability that the trend is significantly different than zero for selected unregulated rivers and streams in the Puget Sound basin.

There were no consistently strong correlations between the annual 7-day average low flow values for the rivers within WRIA 3/4 (Table 2). Calculated annual 7-day average low flow values from Thunder Creek and the Samish River generally correlate weakly with the other rivers within the group used for comparison. Thunder Creek can be classified as a snowmelt-dominated river. The Samish River is a rainfall-dominated river. The other rivers within the group are all transition rivers. It is possible that the different hydrologic regimes partially explain the lack of correlations in low flow.

Table 2. Pearson's correlation coefficient for annual 7-day average low flow for rivers within WRIA 3/4.

Notes: a. Pearson’s r not significantly different than 0 (P>0.05)

The analysis presented above was derived from data in the public domain. The values and trends for 7-day average low flow were calculated from average daily discharge data from fourteen USGS station 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 7-day low flow values were calculated for the period from June 1 – November 1; this time period was chosen to avoid the potential capture of winter low flows in the snowmelt-dominated river system (e.g., Thunder Creek). Trends were determined by calculating the slope of the annual 7-day low flow versus year using simple linear regression. Significance was determined by applying the Student’s t-test to determine the probability of the slope being significantly different than zero (P<0.05).

The significance of the Pearson’s correlation coefficient was determined by estimating the probability that the correlation was different than zero based on the value of the correlation and the sample size. A significant correlation does not indicate a strong correlation.

Analysis of streamflow data revealed decreasing trends in 7-day average low flow values for seven of 14 guaging stations. Among the remaining stations, none showed significant increasing trends. Substantial inter-annual variation in low flow was evident. Annual 7-day average low flows among the river systems in WRIA 3/4 showed no consistent correlation. The weakest correlations were between the snowmelt-dominated (Thunder Creek), the rainfall-dominated (Samish River) and the remaining river systems. Seven-day average low flow could be a useful indicator of changing conditions in these watersheds.

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