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February 22, 2017

Atmospheric rivers are a global weather phenomenon that can bring large amounts of rain or snow to the U.S. West Coast each year. These rivers of wet air form over the Pacific Ocean near Hawaiʻi and pick up large amounts of moisture from the tropics and on their way to the West Coast. This moisture is carried in narrow bands across the Pacific Ocean to California, Oregon, Washington and Nevada.

When a strong atmospheric river (AR) arrives on the west coast of North America, it brings with it a large amount of relatively warm moist air. The precipitation that results when these storms make landfall can result in heavy rains and substantial snowfalls in California, Oregon, Washington, and Nevada.

animation of global satellite images of atmospheric rivers over the Pacific Ocean
Animation of an Atmospheric River event in January 2017 (Credit: NOAA/ESRL/PSD) (Public domain.)

Winter is normally the rainy season in California, but there has not been much rain or snow for several years. Recent AR storms have brought a welcome relief, from a multi-year drought and great conditions for surfers, skiers and snowboarders, but they can also be very hazardous. The intensity of a single, very strong storm can trigger floods, landslides, avalanches, coastal erosion, and debris flows that endanger lives, property and fragile ecosystems. Typically, ARs are strongest in winter and early spring. Over the last month, California, Oregon and Washington are feeling the full force of a protracted sequence of AR winter storms arriving from over the Pacific Ocean.

Although the U.S. Geological Survey (USGS) doesn’t directly forecast the weather, our sister agency, the National Oceanic and Atmospheric Administration (NOAA), via its National Weather Service (NWS) does. The USGS studies and monitors the effects of extreme weather phenomena and long-term climate changes across the U.S. and globally. In particular, the USGS monitors streamflow, river levels, reservoir elevations, rainfall, floods, landslides, erosion, high sea-level stands and many other weather-related earth processes that affect communities. The USGS closely monitors environmental conditions to provide much needed data to the NWS in its responsibilities for hazard warnings, flood alerts and to assist communities across the country in their preparation, response and recovery activities.

The USGS works closely with other federal agencies and cooperative partners to provide real-time scientific data on rivers and streams that is crucial to mitigating hazards associated with floods. USGS streamgages provide long-term, real-time streamflow information. The USGS also compiles historical information on streamflows to assist water managers in the West in water-supply regulation and planning. Historical data also provide the earliest clue on when and where flooding may occur.

The USGS is on the scientific front line in studying the effects of recent atmospheric-river storms. Read further to learn about some of the many ways USGS science helps communities remain resilient in the face of natural hazards.

The ARkStorm Scenario

In 2010, the USGS developed a scenario, called ARkStorm, that brought together almost 100 professionals in many disciplines, including science, economics, and resource management to create and then explore a realistic story about the huge impacts of an AR-driven storm. Although fictional, the scenario was based on real historic events, and is scientifically plausible giving a realistic picture of what local authorities could expect from the most powerful atmospheric river storms that can hit the West Coast. Local officials, resource managers, emergency managers and first responders have used the ARkStorm scenario to prepare for and mitigate the expected effects of a large AR sequence of storms.


report cover: image of atmospheric river approaching US west coast with overwritten text
Report Cover from USGS Open-File Report 2010-1312, "Overview of the ARkStorm Scenario."(Public domain.)
sepia-colored poen and ink drawing of people in boats floating down a city street surrounded by buildings.
Artist's drawing of flooded streets in Sacramento, California (view up K Street from the levee) during the flood of 1862. (Public domain.)




















Hydrologic monitoring and streamgages during storms

The USGS monitors the nation’s waterways with more than 9000 streamgages across the country. These streamgages let the emergency managers and public know how much water is coming down a river or stream; the gages are the front line in providing high water warnings while keeping track of floods and inundation that can be caused by storms. The USGS works to ensure that streamgages in the West and across the country are fully operational during extreme weather conditions.

As multiple atmospheric rivers continue to bring heavy rainfall and snow to many parts of California this month, the California Department of Transportation has reported numerous road closures around the San Francisco Bay Area due to flooding and in the Sierra Nevada due to winter conditions. Traffic and road closures have been a challenge for many USGS crews, but they manage to venture out during the storms to collect additional data and to ensure that the data are accurate by performing verification measurements during high-water flows.

Streamgages can be damaged from fast moving flood and debris flows, but so far, only minor damage to instruments has occurred this year from the high flows. The USGS coordinates streamflow measurements ahead of storms with federal, state and local cooperating agencies. USGS flood-response crews will be out again all winter and spring as rainfall continues, snowpacks melt, rivers rise and flooding occurs.

Data collected by USGS streamgages are used to help safeguard lives, protect property, and inform decision making during emergencies such as floods, and provide vital information needed for human health and safety. The USGS Streamgaging Network is the Nation’s first defense in monitoring potential hazards like flooding, flash flooding and landslides. When rivers exceed or are expected to exceed flood stage, USGS crews travel to streamgage stations to measure how fast water is moving at a cross-section of the river so that discharge can be accurately calculated. Streamflow or discharge is the amount of water flowing in a stream or river, expressed in cubic feet per second (roughly equivalent to the size of a basketball).


man in yellow shirt and camo coveralls standing in thigh-high water with instrument.
USGS technician measuring streamflow in the Sisquoc River near Garey, California, upstream of the Santa Maria Mesa Road Bridge. (Public domain.)
scientific instruments and platform next to river surrounded by snow
 USGS Streamgaging equipment and cableway on the Pit River near Canby, California operates year round.(Public domain.)















Over half of USGS streamgages in California remained above the 90th percentile of normal flow in the third week of January. Since this storm system began, 112 USGS streamgages in California have recorded top 10 peakflows, with more than half (58) of those gages having 20-plus years of record, and over a quarter (29) of those gages having 50-plus years of record. Several crews have measured streamflow at sites that have not had measurable flows in several years due to the drought. For example, discharge was measured recently at site 11152500, on the Salinas River near Spreckels, CA, which is flowing for the first time since October 2013.

Increased turbidity and high suspended-sediment loads (muddy water) caused technical measurement challenges at many sites, requiring crews to rely on mechanical meters, or indirect flood measurements, instead of acoustic meters, such as acoustic Doppler current Profilers or velocimeters, which can be limited by heavy sediment.

man in yellow slicker standing in rushing river up to his thighs, holding a stick-like instrument in the water
USGS hydrographer measuring streamflow using a handheld Acoustic Doppler Velocimeter in fast moving floodwater Cajon Creek near Keenbrook, California. (Public domain.)
two men in camo-patterned overalls standing next to a boat on the shore of a river with an instrument on the ground.
USGS technicians prepare to board a boat to go out to the center of the channel in the San Joaquin River below Friant Dam to get an accurate measurement of water flow during releases from the Dam for flood control. (Public domain.)
Two technicians in a cablecar over a river, with an instrument hanging below them
USGS technicians preparing for a streamflow measurement by lowering an acoustic Doppler current profiler from a cable way on the American River near Fair Oaks, California. (Public domain.)

Currently, 9 out of the 500 plus streamgages operated by the USGS California Water Science Center have sustained damages or are experiencing data interruptions as a result of recent storms. Cooperators have been notified and crews are working on repairs, but streamgages with more extensive flood damage may require additional work and time to restore operations.


Another series of atmospheric river storms will bring rain and snow to California possibly continuing through the end of next week (Feb 23). Precipitation is expected for all parts of California, with the storm tomorrow expected to bring as much as 4-6 inches of rain to Los Angeles and Santa Barbara. NWS issued flash flood warnings for much of southern California for Friday, Feb 17. Major flooding is expected, along with increased potential for flash floods, landslides, and debris flows, especially in recent wildfire burn areas.

Many counties remain under flood warning or flood watch, with much of California’s central valley under a 7-day hazardous weather outlook. The NOAA/NWS California Nevada River Forecast Center reports many California rivers remain above flood (14 gages) or monitor (34 gages) stage today, including 13 gage locations along the Sacramento River, 4 gages on the San Joaquin River, and 4 gages on the Feather River.

The USGS California Water Science Center (CAWSC) has crews responding to this event. The crews finished deploying 25 water-level monitors along the Feather River floodplain below Oroville Dam to collect data that may be used to advance and refine hydrologic models. These data are collected as part of a short-term network that will be used by the USGS to monitor the watershed for the remainder of California's rainy season. The USGS is coordinating this work with officials from the California Department of Water Resources (DWR). Although the mandatory evacuation for the Oroville, CA area has been lifted, an evacuation warning remains in effect and the USGS is following standard safety protocols while working in the area.

10-second video of rushing water in a stream. Person in a small car suspended from overhead cableway, instruments hanging below
USGS technician using an acoustic doppler current profiler to measure streamflow while suspended from a cableway over the Napa River near Napa, California. (Public domain.)


In Nevada, during the first week of January, the National Weather Service issued a flood watch for the greater Lake Tahoe area and western Nevada. A rather warm atmospheric river storm moved into the area resulting in high snow levels and torrential rain in some places. The rain, along with melting snow, could not be absorbed because the ground was already saturated from previous storms. The resulting runoff has caused flooding throughout the Lake Tahoe basin and western Nevada. USGS Nevada Water Science Center hydrologists and hydrologic technicians have been working to make streamflow and water-quality measurements at USGS gaging stations throughout the area.

Woman standing in middle of stream next to instruments
USGS technician measuring streamflow in northern Nevada. (Public domain.)
Gray metal box standing next to a river with a sign in green and black writing
USGS Streamgaging Station in northern Nevada. (Public domain.)
Large ruler-like instrument under a bridge standing up in the river water
USGS staff gage (ruler) measuring the height of the water, on the Carson River, near Brunswick Canyon, Nevada. (Public domain.)















Oregon and Washington

USGS field crews across the Pacific Northwest have spent the last few weeks measuring streamflow and closely monitoring river and stream conditions. The current storms have brought both snow and rain but high water and damage have been much less severe than previous floods, such as those in late 1964 and early1996. However, like those previous storms, USGS streamgagers were out collecting high flow measurements and water quality samples at numerous sites throughout the region. High water was concentrated along the coast and centered around the Portland and Medford metro areas.

The Christmas flood of 1964 encompassed about 200,000 square miles, roughly the size of France, and resulted in 47 deaths, left thousands homeless and caused more than $540 million ($3.9 billion today) worth of damage. Areas in Oregon, Idaho, California, Washington and Nevada experienced record-breaking floods caused by three storms between Dec. 19 and Jan. 31, 1964.

The Christmas flood of 1964 was driven by atmospheric rivers or “pineapple express" storms that battered the region producing as much as 15 inches of rain in 24 hours at some locations. The combination of heavy rain, melting snow, and frozen ground caused extreme runoff, erosion and flooding. The flood caused record-breaking peak streamflows, transported large amounts of sediment, and inflicted extensive flood damage. However, in many areas storage in reservoirs and operation of flood-control facilities prevented far greater damage.

Flood managers rely on USGS streamflow data to make time-critical decisions to protect life and property when severe flood conditions arise. Most USGS streamgage stations across the nation are equipped to transmit information in real time to local, state, and national emergency management and warning agencies such as the National Weather Service and the U.S. Army Corps of Engineers. Real-time streamgage data are also available to the public online. USGS provides streamflow data so that the public has the earliest possible notice of an impending flood. Maintaining these stations, and the flow of information they provide is central to the USGS Mission.


Mapping changing beaches

Following the AR storms in early January, USGS scientists in Santa Cruz, California surveyed local beaches and ocean bottom adjacent to river mouths to compile a three-dimensional map of how the coast changed during storms that struck the previous week. They used high-precision GPS receivers carried on foot and mounted on ATVs to measure beach elevations (topography). They used GPS receivers and 200-kilohertz echo sounders (sonar) mounted on a boat and personal watercraft to measure underwater elevations (bathymetry) on transects along the shore.

Large waves crashing on rocks at beach.
Large storm waves crashing on the rocks near Santa Cruz, California(Credit: Christine Hegermiller, U.S. Geological Survey. Public domain.)
man walking along wet sand on beach wearing a backpack with an antenna sticking pout of it
USGS scientist Daniel Hoover mapping the beach at Santa Cruz with a GPS-equipped backpack unit.(Credit: Andrew Stevens, U.S. Geological Survey. Public domain.)












two men standing in a boat tied up to a dock
USGS scientists readying a sonar-equipped boat to map the ocean bottom near Santa Cruz, Calif.(Credit: Andrew Stevens, U.S. Geological Survey. Public domain.)


person standing up riding a personal watercraft across small ocean swells, with buildings and a long pier in the background
A sonar-equipped personal watercraft mapping the bathymetry underwater near Santa Cruz, Calif.(Credit: Andrew Stevens, U.S. Geological Survey. Public domain.)




























A terrestrial lidar scanner was added to the mix of instruments. Lidar (light detection and ranging) uses laser light to measure distances and produce highly accurate three-dimensional maps and images of terrain. It is similar to radar but uses laser light instead of radio waves. This instrument rotates 360 degrees and bounces a low-power laser beam safe for the naked eye off everything around it. By measuring the length of time it takes for the light to bounce off an object and return to the scanner, the scanner can capture an accurate three-dimensional measurement of the surrounding surfaces. It is capable of doing this as fast as 122,000 times each second and produces about 10 million points of data in a single rotation.

man in gray sweatshirt standing next to an instrument on top of a large yellow tripod.
USGS geographer Josh Logan prepares to measure beach elevations with a lidar (light detection and ranging) scanner near Capitola on December 10, 2014, in anticipation of a large storm forecast for the next day. (Credit: Amy West, U.S. Geological Survey. Public domain.)
two young men standing on a pier with a large tripod, looking down at camera, and holding equipment.
 USGS scientists setting up a lidar scanner on the pier to map the beach near Capitola, California.(Credit: Andrew Stevens, U.S. Geological Survey. Public domain.)
















The recent series of AR storms in central California had a big effect just offshore the Santa Cruz Municipal Beach at the mouth of the San Lorenzo River. In particular, they found that significant amounts of sediment that had built up in the watershed after so many years of drought, washed down the San Lorenzo River, building a large offshore sand bar the equivalent of about 10,000 dump trucks worth of sand. The presence of the temporary sand bar delighted local surfers, because of the large waves breaking over it.

This current fieldwork complements semi-annual surveys over a larger area, from Santa Cruz to Moss Landing, that the USGS began in October 2014 to document the patterns and volumes of sand movement along the coast. Conducting surveys over many years will ultimately provide a detailed picture of how our coastline reacts to changes in waves and sand input. The results can be incorporated into future scenarios of sea-level rise and climate change, contributing directly to Monterey Bay communities working on how to protect their coastlines. The USGS is in the process of building a model to assess the impacts of climate change across the region that will be available in 2018.

Targeted surveys of vulnerable and dynamic coastal zones such as the mouth of the San Lorenzo River and the Capitola area after large winter storms will enable scientists to better understand how the coast is shaped and responds to extreme events, such as those occurring during storms from atmospheric rivers. Sandy coastlines are a valuable resource that protect communities and infrastructure from wave impacts, serve as habitat for important species, and provide a variety of recreational opportunities. Big storms that wreak havoc in the area may also do some good by helping streams carry much-needed sand to local beaches. More information about this research is online.

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