1983 vs 2023 Spring Flooding

1983 vs 2023 Spring Flooding, Ashley Creek Flowing Underneath Highway 191, Vernal, UT, Courtesy & Copyright Holly Strand, Photographer
Ashley Creek Flowing Underneath Highway 191, Vernal, UT
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Courtesy & Copyright Holly Strand, Photographer

Hi, I’m Holly Strand from the Utah Division of Emergency Management.

The roar you hear in the background is Ashley Creek flowing underneath Highway 191 in Vernal. Rivers and creeks all over the state have been running high with melted snow from the biggest snowpack ever recorded in Utah.

1983 was last time we had anything near this amount of snowmelt fueling our spring runoff. That year, there were multiple 100-year (and bigger) floods. The landslide in Thistle triggered Utah’s first presidential disaster declaration. For weeks, floods, landslides, and debris flows damaged homes, highways, and drainage canals. 22 of Utah’s 29 counties were affected.

This year—with a historic snowpack– it seems like we should be experiencing more 100-year floods or even 500-year floods. Even though only 40 years have gone by since 1983. It’s quite possible, because 100-year floods don’t happen just once every 100 years. They are just projected to happen every 100 years on average**

So did this year’s historic snowpack—more than in 1983–cause another round of 100-year floods? Hasn’t so far and doesn’t look like it will. There has been isolated flooding in some areas, and groundwater flooding, but we just aren’t seeing damage on the same scale that we saw 40 years ago. Why not?

For one thing, Utah communities took lots of mitigation measures in the wake of the 1983 floods. New debris basins were built, stormwater systems improved, river and watersheds were restored, and at-risk structures were flood-proofed. Communities also prepared their residents for floods with information and sandbags! But a big reason Utah didn’t suffer from 100-year or floods, is that the 2023 spring was absolutely ideal for moving water safely from the snowy, high elevations to the thirsty reservoirs and lakes below. Since peak snowpack in the beginning of April, there’s been a long gradual warming trend with no big storms. Stream flows have been consistently high for 2 months instead coming down all at once during a sudden long hot spell of 90 plus temperatures. We dodged a bullet this year thanks to mitigation, preparation and Mother Nature!

Thanks to Glen Merrill with the National Weather Service in Salt Lake City and to Tom Wright, hydrological engineer at AECOM for their expert knowledge.

I’m Holly Strand and I’m Wild About Utah and Utah Public Radio!

**A 100-year flood–has a 1% chance of happening each and every year on any given stream reach. Consider what happens when you roll dice. Let’s say you roll a double six. You could get double sixes on the very next roll.  But over many, many, many rolls, you will average double sixes only once every 36 rolls. Similarly you could get a 100-year flood one year, and then the very next year you could roll another 100-year flood but there’s a 99% chance that you won’t. But averaged over a long time, like 1000s of years, a 100-year flood should happen—on average—once every 100 years. To make this concept clearer flood experts prefer to say 1% annual chance flood instead of 100-year flood.


Images: Courtesy & Copyright Holly Strand, Photographer
Additional Audio: Holly Strand
Voice: Holly Strand
Text: Holly Strand, Utah Division of Emergency Management
Additional Reading: Holly Strand

Sources & Additional Reading

Wild About Utah pieces authored by Holly Strand

Some interesting sites for flood nerds:
Utah Flood Hazards and Floodplain Management, Utah Division of Emergency Management, https://floodhazards.utah.gov/

Colorado Basin River Forecast Center, National Weather Service, National Oceanic and Atmospheric Administration, US Department of Commerce, https://www.cbrfc.noaa.gov/lmap/lmap.php?interface=snow

NOAA ATLAS 14 Point Precipitation Frequency Estimates: UT, Office of Water Prediction (OWP), National Weather Service, National Oceanic and Atmospheric Administration, US Department of Commerce, https://hdsc.nws.noaa.gov/pfds/pfds_map_cont.html?bkmrk=ut


Ashley Creek Flowing Underneath Highway 191, Vernal, UT
Courtesy & Copyright Holly Strand, Photographer

Snow Pack Dynamics

Snow Layers
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Forest Service Avalanche Center
Jim Conway, Graphic Artist
Depth Hoar
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and a deformation animation
Forest Service Avalanche Center
Jim Conway, Graphic Artist

Water is our planet’s magical molecule, changing states faster than a presidential candidate. Snowpacks vaporize, ice melts and re-freezes, lakes evaporate, and cooled water vapor condenses back as clouds, snowflakes and hoarfrost. The muffled silence of the winter snowpack belies its dramatic pace of transformation.

In his book entitled “Life in the Cold”, author Peter Marchand explains the dynamic nature of the snowpack. Within a few hours after a snow storm, destructive metamorphism sets to work on the newly fallen snow. The delicate crystalline structure of each snowflake is quickly degraded. The intricate flakes transform to amorphous icy grains. Wind, warmth and compression accelerate destructive metamorphism, leaving a firmer, denser snowpack. At the surface, not only does snow strongly reflect the weak warmth of winter sunlight, but on a clear night, it radiates energy, greatly cooling the surface.

Meanwhile, the soil beneath the snowpack is typically warmer than the overlying snow, which is why springs can run all winter long. Three feet underground, soil temperature is within a few degrees of that location’s average annual air temperature. Sandwiched between the warm soil and the cold air, the blanket of snow is a great thermal insulator; fresh snow is the equal of fiberglass insulation. As a result, soil warmth transforms snow deep under the snowpack into water vapor. This moisture spreads through air spaces in the snowpack, following the thermal gradient to the chilly snow surface. As the moisture vacates the lower layers, a brittle porous layer develops in the snowpack. Termed “depth hoar”, it is weak, icy and prone to collapse. When the heavy overlying snowpack shifts, the crumbly depth hoar can release an avalanche, a powerful reminder of snowpack transformations for any backcountry traveler.

Come spring, every particle of Utah’s snowpack undertakes its final transformation. Some sublimates to waft away on warm springtime winds. Most of it melts away to feed the groundwater, springs and streams that give us cool relief on a hot summer day and provide the precious water that every Utahn depends on.

This is Linda Kervin for Bridgerland Audubon Society.


Graphics: Courtesy Forest Service Avalanche Center, https://www.fsavalanche.org/
Text: Jim Cane, Bridgerland Audubon Society

Additional Reading:

Life in the Cold by Peter Marchand:https://www.upne.com/9619460.html

Forest Service National Avalanche Center, Avalanche Awareness Website: https://www.fsavalanche.org/

Depth Hoar: https://www.fsavalanche.org/encyclopedia/depth_hoar.htm

Utah Avalanche Center: https://utahavalanchecenter.org/