Summer low flow in streams and rivers

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).

Monitoring Program: Streamflow Monitoring Program, Washington Department of Ecology

Data Source: U.S. Geological Survey Groundwater and Streamflow Information Program - Streamgaging Network, compiled by the Streamflow Monitoring Program at the Washington Department of Ecology

Summer flows in streams and rivers occur at the time of year characterized by warm temperatures, little rainfall, and depleted snowpacks. This coincides with the time when water demands are greatest, despite that supply is lowest. The USGS Groundwater and Streamflow Information Program collects streamflow data through a network of streamgages that continuously monitor streams, which report mean daily flows year-round and are available online. From these streamflow records, 7-day mean daily streamflows were computed and serves as the basis for this analysis.

This indicator reports on the number of days during the summer period (July 15 through October 31) that a stream’s 7-day mean daily flow falls below “normal” relative to the baseline data for that river. Normal flows are defined by conditions observed over a 50-year baseline, from 1948 to 1998.