When flows in streams and rivers diminish, 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, helps resource managers and communities create 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 and other species at various life stages in Puget Sound streams and rivers. Low flows have the potential to impact habitat capacity for rearing juvenile salmon and can prevent access to, and reduce the availability of, adult spawning areas.

• The occurrence of below normal summer flows is increasing in unregulated streams and rivers across Puget Sound. In 2023, summer flows were below normal most of the time at 16 of the 18 indicator gages (see Interpretation of Results for more details). Thus, we determine this indicator to be GETTING WORSE.
• Consecutive years with below normal summer flows have increased since 1985 and especially since 2015. Since 2015, most of the rain, transitional, and snow-fed watersheds had below normal summer flows over 60% of the time.
• Accelerated glacial melt may temporarily offset diminishing low flows in some rivers. Summer flows are enhanced by glacier meltwater at four of the indicator gages. These systems have generally fewer occurrences of below normal flows during the summer flow period.
• Flows below 1948-1998 baseline minimums were observed at least once at all indicator gages during the 1999-2023 study period.  The occurrences of flows below baseline minimums have increased since 2015. In 2023, 7-day mean daily flows below baseline minimums occurred greater than 50 percent of the time at 9 of the 18 gages in the analysis.  
• Supporting analysis shows that the timing of streamflow is changing in Puget Sound. Between 1948 and 2023, 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 the status of summer flows at a given gage for each year. Gages are noted as rain-sourced, transitional, or snow-sourced according to their Center of Timing (CT) date.  Gages in each of the three categories show an increasing trend in the percentage of days where flows are below normal since 1985. 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 shaded cells from left to right. Beginning in 2015, we see consistent consecutive years where nearly all gages had below normal 7-day summer flows over 50% of the time, indicated by the purple cells. The dark purple cells indicate below normal flows occurring greater than 75% of the time. In 2023, 11 of the 16 gages showed 7-day mean daily flows were below normal greater than 75% of the time.

Summer flows may be enhanced by glacier meltwater at two transitional and two snow-dominated indicator gages (denoted with an asterisk (*) in Figure 1). These watersheds with substantial glaciers in the basin headwaters have generally fewer days with below normal flows compared to the other watersheds. The more stable flows observed at these gages are likely a temporary boost to summer flows from accelerated glacial melt.

Figure 1. Percent of days between July 15^th and October 31st where the mean daily flow was below normal (i.e., below the 1948-1998 baseline 25^th percentile) each year, by streamgage.

Flows below the minimum observed over the 50-year baseline, were recorded at least once at all indicator gages during the 1999-2023 study period (Figure 2). The total count of years between 1999 and 2023 where flows were below the baseline minimum ranged from just one year (rain-dominated Huge Creek near Wauna (12073500)) up to 13 of the 25 years analyzed (transitional North Fork Snoqualmie (121420000), and North Fork Skokomish Rivers (12056500) ). Figure 2 shows all the analyzed streams, except Huge Creek, recorded 7-day flows below the baseline minimum in 2023. In 2023, summer flows falling below the baseline minimum occurred greater than 50% of the time at half the streams analyzed. 

The streamgage at Huge Creek near Wauna is unique in that it represents flows in a basin 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 the only basin with significant groundwater input and was not subject to below normal summer flows in recent years as seen in other Puget Sound watersheds.

Figure 2. Percent of days between July 15^th and October 31st where the 7-day mean daily flow was below the 7-day minimum flow observed over the 1948-1998 baseline period.

Supporting Analysis – Changes in Streamflow Timing

Key Center of Timing Results

• Trend analyses show with certainty (α<0.10) that the center of timing (CT) occurred earlier in the year at 10 of the 16 snow-dominated and transitional gages from 1948 to 2023. 
• All four of the snow-dominated streams, including the two with significant glaciers in their headwaters show, with certainty, earlier CT.
• While all 12 of the transitional streams show earlier CT, six do not show earlier CT with certainty (α>0.10).
• The six transitional streams not showing certainty in earlier center of timing include two streams with glaciers in their headwaters (Puyallup River nr Electron (12093500), and Nisqually River nr National (12082500). 
• Between 1999 and 2023, the average CT at the snow-dominated and transitional gages was 7 days earlier compared to the 1948 to 1998 period (Table 2.)
• Between 1986 and 2023, the average CT at the snow-dominated and transitional gages was 12 days earlier compared to the 1948 to 1985 period (Table 3). 
• In contrast, between 2015 and 2023, the average CT at the snow-dominated and transitional gages was 23 days earlier compared to the 1948 to 1985 period (not shown).

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 2023 (or period of record available) for all 18 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 (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 16 snow-sourced or transitional gages and the two rain-sourced gages. To test whether CT changed between 1948 and 2023, we conducted a Theil-Sen slope trend analysis on the 16 snow-sourced or transitional gages. In addition, we compared average CTs across multiple time periods. 

Center of Timing Results and Interpretation

Table 2 compares the average CT from 1948-1998 and 1999-2023. CT occurred an average of 7 days earlier over the latter period. 

Recognizing the overall increase in occurrences of lower than normal 7-day mean daily flows beginning in the mid- 1980s, Table 3 compares the average CT from 1948-1985 and 1986-2023. For the 16 snow-sourced and transitional gages, CT occurred on average 12 days earlier over the 1986-2023 period compared to the 1948-1985 period.

Trend analyses show with certainty (α <0.10) that CT is occurring earlier in the year at 10 of the 16 snow-dominated and transitional gages from 1948 to 2023 (Table 4). Neither of the two  rain-sourced basins showed a significant change in the CT date over time.

Both the trend and comparative analyses suggest that CT in many Puget Sound streams and rivers are regressing to earlier occurrences over time, in snow-sourced and transitional systems. This means the low flow season in our region may become 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).

Additional analysis not central to the indicator revealed annual cumulative flows (not shown) declined at 11 of the 16 snow-sourced or transitional gages analyzed from 1948 to 2023; however, none of the records showed a significant trend (α < 0.10) in decreasing or increasing cumulative flows.

The presence of glaciers and their melting may cause different responses to 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.

Table 2.

** Greenwater R. at Greenwater missing cumulative flow data from 1978-1993.

*Substantial glaciers in headwaters.

Table 3.

** Greenwater R. at Greenwater missing cumulative flow data from 1978-1993.

*Substantial glaciers in headwaters.

Table 4.

** Greenwater R. at Greenwater missing cumulative flow data from 1978-1993.

*Substantial glaciers in headwaters.

Numbers in bold signify trend is significant (α < 0.10)

Georgiadis, N., Bogue, K., DeGasperi, C., 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., Luce C. H., Wenger, S. J., and Berghuijs, W. R., 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.

Mantua, N.J. 1999: The Pacific Decadal Oscillation. A brief overview for non-specialists, Encyclopedia of Environmental Change.

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

Vannest, K.J., Parker, R.I., Gonen, O., & Adiguzel, T. (2016). Single Case Research: web-based calculators for SCR analysis. (Version 2.0) [Web-based application]. College Station, TX: Texas A&M; University. Accessed 2/11/2025. https://singlecaseresearch.org/calculators/theil-sen/.