When water in streams and rivers goes down, it places pressure on municipal, residential, industrial, and agricultural water supplies. Keeping track of flows in the summer, the driest and warmest time of year, and the causes for change, helps resource managers and communities formulate strategies that help to minimize or mitigate declining flows where possible and respond to climate change.

Below normal summer flows also adversely affect fish and wildlife habitats. During the period of summer low flows, there are several salmon species and life histories living in Puget Sound streams and rivers. When flows are too low, it has the potential to impact habitat capacity for rearing juvenile salmon and to prevent access to, or the availability of, adult spawning areas.

Climate change impacts summer streamflows. With rising snow elevations and less snowpack, summer low flows are becoming lower and longer lasting throughout the region (see State of Salmon in Watersheds, Water Quantity Risk Factors).  

• The occurrence of below normal summer flows is increasing in unregulated streams and rivers across Puget Sound. In 2021, summer flows were below normal most of the time at three-quarters of the indicator gages (see Interpretation of Results for more details).
• Consecutive years with below normal summer flows have increased since 1985 and especially since 2015.
• Since 2015, most of the rain, transitional, and snow-sourced systems had below normal summer flows over 75% of the time.
• Accelerated glacial melt may temporarily offset diminishing low flows in some rivers. Substantial glaciers are present around basin headwaters of five of the indicator gages. These systems have generally maintained stable summer streamflows, at levels similar to the 1948-1998 baseline period.
• Daily flows below 1948-1998 baseline minimums were observed at all but one indicator gage (Huge Creek near Wauna) during the 1999-2021 study period and especially since 2015.
• Supporting analysis shows that the timing of streamflow is changing in Puget Sound. Between 1948 and 2021, the center of timing (CT) at most indicator gages regressed to earlier occurrences over time. This means the low flow season in our region is becoming longer as larger fractions of total annual runoff occur progressively earlier in the year (see Interpretation of Results – Changes in Streamflow Timing for more details).

Figure 1 shows a chart of daily summer flows between July 15 and September 15 in 2021 (black line) relative to the 1948-1998 baseline range for each of the 19 indicator gages that met our criteria. Flows lower than the baseline 25^th percentile (yellow band and below) are interpreted as “below normal”. When flows in a given year are consistently below the 25^th percentile, we conclude that flows that year were regularly below the normal range. In 2021, summer flows were below normal most of the time at about three-quarters of the 19 selected gages.

Figure 1. Daily summer flows in 2021 (black line) relative to the 1948-1998 baseline range for each of the 19 indicator gages. Click the image to view a full size figure.

Figure 2 shows the status of summer flows at a given gage for each year. Systems are grouped by their Center of Timing (CT) date as rain-sourced, transitional, or snow-sourced. Systems under each of the three categories show an increasing trend in the percentage of days where flows are below normal. After 1985, these below normal flows occur more consistently across the region and occur over consecutive years, indicated by a shift from predominantly blue cells to purple cells from left to right. Beginning in 2015, we see consecutive years where nearly all systems had below normal summer flows over 75% of the time, indicated by the dark purple cells.

Summer flows are enhanced by glacier meltwater at three transitional and two snow-sourced indicator gages (denoted with an asterisk (*) in Figure 2). These systems with substantial glaciers in their basin headwaters have fewer days with below normal flows compared to the other rain, transitional, and snow-sourced rivers. The more stable flows observed in these systems is likely a temporary offset from accelerated glacial melt.

Figure 2. Percent of days between July 15th and September 15th where the mean daily flow was below normal (i.e., below the 1948-1998 baseline 25th percentile) each year, by streamgage. Click the image to view a full size figure.

Flows below the lowest flow observed over the 50-year baseline, were recorded at all but one indicator gage (Huge Creek near Wauna, Site Code 12073500) during the 1999-2021 study period (Figure 3). The streamgage at Huge Creek near Wauna represents flows at a system that is smaller than others measured for the indicator (6.5 sq mi drainage area compared to an average area of 303.4 sq mi). Huge Creek is also the only system with a significant groundwater input and was not subject to below normal summer flows in recent years as seen in other Puget Sound systems.

The count of years between 1999 and 2021 where flows were below the baseline minimum ranged from 2 years (rain-sourced Taylor Creek near Selleck, Site Code 12117000) up to 14 of 23 years (snow-sourced Sauk River near Sauk, Site Code 12189500). In 2015, summer flows at all but two of the non-glacier enhanced systems were below the baseline minimum over half the time.

Figure 3. Annual percent of days between July 15th and September 15th where the mean daily flow was below the minimum flow observed over the 1948-1998 baseline period. Click the image to view a full size figure.

Supporting Analysis - Changes in Streamflow Timing

Key Center of Timing Results

• Trend analyses show with certainty (p<0.10) that the center of timing (CT) is occurring earlier in the year at 10 of the 13 snow-sourced and transitional gages from 1948 to 2021. The three gages not showing significant earlier timing have substantial glaciers in their headwaters.
• Between 1986 and 2021, the average CT at the snow-sourced and transitional gages was 13 days earlier compared to the 1948-1985 period. The average CT at rain-sourced gages was roughly 2 days earlier.

Center of Timing Approach

In addition to reporting on status and trends in summer low flow, we explored changes in streamflow timing. Center of timing (CT), also known as center of mass, or center of volume, is the date in the water year (October 1 to September 30) when half of the cumulative flow occurs. The cumulative flow is the total volume of water passing a gage in an entire water year.

CT tells us how flows are distributed over a water year and correlate with the type of stream (Stewart et al. 2005). For example, a later CT indicates a higher proportion of the flows come from snowmelt; earlier CT indicates a higher proportion of the flows come from rainfall or that snowmelt is occurring earlier in the year.

We calculated the annual CT from 1948 to 2021 for all 19 indicator gages. We used the approach described in Kormos et al. (2016), to classify each streamgage based on the mean annual CT from 1948 to 1998 (the baseline period) as follows:

• Snow-sourced systems: mean CT occurs after April 18 (after water year Julian day 200)
• Transitional systems (between snow and rain-sourced): mean CT occurs between February 28 to April 17 (between water year Julian day 150 to 200)
• Rain-sourced systems: mean CT occurs before February 27 (before water year Julian day 150)

We calculated CT for each of the 13 snow-sourced or transitional gages and the six rain-sourced gages. To test whether CT changed between 1948 and 2021, we conducted a trend analysis on the 13 snow-sourced or transitional gages and compared average CT across multiple time periods.

Center of Timing Results and Interpretation

Table 2 compares the average CT from 1948-1998 and 1999-2021. CT occurred on average 7 days earlier during the latter period.

Recognizing the significant global climate shift that occurred in the mid- to late-1980s, Table 3 compares the average CT from 1948-1985 and 1986-2021. For the 13 snow-sourced and transitional gages, CT occurred on average 13 days earlier over the 1986-2021 period compared to the 1948-1985 period. Changes in the CT at rain-sourced gages varied and was on average 2 days earlier during the latter period.

Trend analyses show with certainty (p<0.10) that the CT is occurring earlier in the year at 10 of the 13 snow-sourced and transitional gages from 1948 to 2021 (Table 4). Only two of the six rain-sourced systems showed a significant change in the CT date over time, with one site indicating the occurrence shifted earlier in the year and the other indicating the occurrence had shifted later in the year.

Both the trend and comparative analyses suggest that overall, CT in Puget Sound streams and rivers is regressing to earlier occurrences over time, especially in the snow-sourced and transitional systems. This means the low flow season in our region is becoming longer as larger fractions of total annual runoff are occurring progressively earlier in the year.

Recent studies indicate earlier CT dates are mostly related to warmer temperatures and reduced snowpack. (Kormos et al. 2016; Georgiadis et al. 2022). Warming winter and spring temperatures force more precipitation to fall as rain rather than snow, and further hasten earlier snowmelt (Stewart et al. 2005).

Annual cumulative flows (not shown) declined at 9 of the 13 snow-sourced or transitional gages analyzed from 1948 to 2021. However, none of the gages showed a significant trend in decreasing or increasing cumulative flows. Three of the four gages with increasing cumulative flows were from the Puyallup and Nisqually River basins with substantial glaciers at their headwaters. The fourth gage, showing a slight increase, is the North Fork Nooksack River near Glacier which also has glacial input. 

The presence of glaciers and their melting may cause different responses on streamflows than that of non-glacial basins (Stewart et al. 2005). We see this distinction quite clearly between streams with substantial glacial inputs and those without, both in the CT and summer flows analyses.

References

Georgiadis, N., K. Bogue, C. DeGasperi. 2022. Distinguishing climate change impacts from development impacts on summer low flows in Puget Sound streams. Puget Sound Institute. University of Washington.

Kormos, P.R., C.H. Luce, S.J. Wenger, W.R. Berghuijs. 2016. Trends and sensitivities of low streamflow extremes to discharge timing and magnitude in Pacific Northwest Mountain streams. Water Resources Research, 52, 4990–5007. DOI:10.1002/2015WR018125

Reid, P.C., R.E. Hari, G. Beaugrand, D.M. Livingstone, C. Marty, D. Straile, J. Barichivich, E. Goberville, R. Adrian, Y. Aono, R. Brown, J. Foster, P. Groisman, P. Helaouet, H. Hsu, R. Kirby, J. Knight, A. Kraberg, J. Li, T. Lo, R.B. Myneni, R.P. North, J.A. Pounds, T. Sparks, R. Stubi, Y. Tian, K.H. Wiltshire, D. Xiao, Z. Zhu. 2015. Global impacts of the 1980s regime shift. Global Change Biology. DOI: 10.1111/gcb.133106  

Stewart, I.T., D.R. Cayan, M.D. Dettinger. 2005. Changes toward earlier streamflow timing across Western North America. Journal of Climate, Volume 18: Issue 8.