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Streams that keep flowing all summer — "permanent" streams — are vital habitat for aquatic life and an important source of water. In mountain country such as Mount Rainier National Park in Washington, it is natural for some streams to dry up seasonally or to run only after rain or snowmelt. But in years with far less snow than normal, such as 2015, more of them go dry. That leaves less water for the park's infrastructure, with consequences for stream ecology that are not yet known.
A typical flowing stream in late summer. Photo: National Park Service.
A typical dry stream in late summer. Photo: National Park Service.
With snowpack already declining and expected to decline further, the U.S. Geological Survey and the park set out to predict which streams are likely to be dry by late summer.
A model built on simple observations
The model was calibrated with straightforward flow-or-no-flow checks of small streams across the park and the surrounding area, made in late summer of 2018, 2019 and 2020 before the fall rains began. It uses information about geology, topography, climate and land cover to predict whether a stream will be flowing or dry. It was designed to improve on PROSPERPNW, an existing regional model of stream permanence covering several states of the Pacific Northwest.
For each stretch of stream, the model gives a probability from 0 to 100 percent that water will be flowing at the surface in late summer:
| Probability of late-summer flow | Classification |
|---|---|
| above 60 percent | likely flowing |
| 40 to 60 percent | nondeterminate |
| below 40 percent | likely dry |
A nondeterminate stream may be one that flows in some years and dries in others, depending on climate. It may also mark a place where the model cannot capture something important, such as water moving underground beneath the channel.
Many streams go dry each summer
About 35 percent of stream reaches are estimated to go dry at some point in late summer, about 37 percent to keep flowing, and about 28 percent are nondeterminate. Chart: U.S. Geological Survey.
Location matters. Streams that do not flow from glaciers or from high areas of year-round snow are more likely to go dry. In years with less snowpack, streams are expected to dry sooner and stay dry longer into the summer.
Streamflow permanence probability across the park, with the places where flow or no flow was observed. Map: U.S. Geological Survey.
Why dry streams matter
- Less water to use. Dry streams mean less water within reach.
- Less habitat. They limit habitat for animals that live in streams and on land.
- Visitors and park systems. Hikers and other visitors who count on stream crossings for drinking water are affected, and so are the park's surface-water supply systems.
The findings will help the park identify areas where surface water is scarce, especially in low-snow years. The USGS plans to extend the work to better predict when streams go dry and for how long.
Sources
- U.S. Geological Survey, prepared in cooperation with the National Park Service, "Streamflow permanence in Mount Rainier National Park, Washington," Fact Sheet 2023–3051: https://pubs.usgs.gov/publication/fs20233051
- The fact sheet summarizes K.L. Jaeger, R. Sando, S.B. Dunn and A.S. Gendaszek, "Predicting probabilities of late summer surface flow presence in a glaciated mountainous headwater region," Hydrological Processes, 2023: https://doi.org/10.1002/hyp.14813 The photographs, chart and map are reproduced from the fact sheet.
Licenza: CC0 1.0 (pubblico dominio) · Tratto da pubs.usgs.gov
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