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El Niño is a phenomenon that occurs when unusually warm ocean water piles up along the equatorial west coast of South America. When this phenomenon develops, it affects weather patterns around the globe, including the winter weather along the west coast of North America. This unusual pattern of sea surface temperatures occurs in irregular cycles about three to seven years apart.

El Niño is a phenomenon that occurs when unusually warm ocean water piles up along the equatorial west coast of South America. When this phenomenon develops, it affects weather patterns around the globe, including the winter weather along the west coast of North America. This unusual pattern of sea surface temperatures occurs in irregular cycles about three to seven years apart.

El Nino map
The reds and orange colors on this world map indicate warmer sea surface temperatures. The large band of warmer waters along the equator west of South America is characteristic of El Niño. Image credit: NOAA

During an El Niño year, California and parts of the southern U.S. can be subject to a cold, wet winter. Winter is normally the rainy season in California, and during past El Niño winters (especially in really strong El Niño years, like this one), greater-than-usual numbers of storms arrive, one after another, resulting in wetter-than-usual winters with large amounts of rain on the coast and snow in the mountains. Some El Niño years (especially weak to moderate El Niños) result in dryer than average years in California and the West Coast, providing few storms to wet the landscape.

El Niño conditions have frequently brought drier, warmer winters to the northern parts of the continental U.S. Specifically in the Pacific Northwest, an El Niño tends to favor a warmer winter and lower-than-average precipitation. In turn, these conditions tend to favor a thinner winter snowpack and subsequently less meltwater in the spring. An El Niño can potentially create conditions for more severe flooding in the wetter winter months and possibly drier spring and summer months in the Pacific NW. In California, historically some of the heaviest precipitation, and all of the largest flood years have been non-El Niño years.

NOAA U.S. winter outlook precipitation map
During an El Niño winter, the temperatures in the northern U.S. are usually warmer and drier than average, and the southern parts of the continental U.S. are usually cooler and wetter. Image credit: NOAA.
NOAA U.S. winter outlook temperature map
During an El Niño winter, the temperatures in the northern U.S. are usually warmer and drier than average, and the southern parts of the continental U.S. are usually cooler and wetter. Image credit: NOAA.















El Niño weather is not just a single severe storm, but it can increase the number and intensity of storms during a season, triggering floods, landslides, coastal erosion, and damage to fragile ecosystems. These changes may occur as early as October in an El Niño year, but typically are strongest in winter and early spring. Now mid-January, California is starting to feel the full force of these sequential El Niño winter storms.

Although the U.S. Geological Survey doesn’t directly study or forecast the weather (our sister agency, NOAA, and its National Weather Service do), the USGS studies and documents the effects and impacts of long-term climate changes and weather phenomena across the U.S. and globally. In particular, the USGS monitors streamflow, floods, landslides, erosion, sea-level rise, and many other earth processes that affect communities and that are often affected by El Niño weather patterns. USGS closely monitors these effects to assist the NWS in its responsibilities for hazard warnings and to assist communities across the country in their preparation, response, and recovery activities.

In addition to the obvious effects El Niño can have on storms, floods and coastal hazards, it can also affect long-distance migratory birds, the snowpack in the mountains of the western U.S., fire seasons in Alaska and across the continental U.S., the health and distribution of oceanic and freshwater fish, and even very local environments such as the sediment and algae in the waters of San Francisco Bay. The USGS studies and monitors these effects and provides much of the scientific basis for planning and responding to them.

The USGS is on the scientific front line in studying the impacts of El Niño. Read further to learn about some of the many ways USGS science helps communities remain resilient in the face of natural hazards.

Hydrologic monitoring and streamgages

USGS monitors the nations waterways with more than 8000 active continuous streamgages across the country. These streamgages tell emergency managers and the public how much water is coming down a river or stream; they are the front line in keeping track of floods and inundation that can be caused by El Niño storms. USGS works to ensure that streamgages in the West and across the country are fully operational during extreme conditions of an El Niño weather pattern.

Most USGS streamgage stations 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 warning of an impending flood.

Image: El Niño Hits San Diego
Sections of Fashion Valley Road, San Diego, were closed by flooding of the San Diego River. Closed roads were a common scene in San Diego and throughout California during the El Niño-related storms, Jan 6 - 7, 2016. Hydrographers from the Poway Field Office closely monitored high-flow conditions throughout the storms.Location:Fashion Valley Road, San DiegoPublic domain

USGS works closely with other federal agencies to provide real-time scientific information on rivers and streams that is crucial in mitigating hazardsassociated with floods. USGS also compiles historical information on streamflows during past El Niños to assist water managers in the West in water-supply planning. Historical data also provide the earliest clue on where flooding may occur.

In preparation for the expected heavy and frequent rains this winter, hydrologic technicians from the USGS California Water Science Center (CAWSC) have recently installed additional precipitation and stream-stage gages in newly burned landscapes in northern California that have increased risk of hazards such as flooding, flash flooding, and debris flows. Crews from USGS field offices installed six new gages in the Valley Fire burn area and five in the Butte Fire burn area. The real time data feeds are displayed to the public on our website for each of these sites. These additional gages will help provide early warning of potential threats at downstream areas.

Not only does USGS install gages and remotely monitor the data collected, USGS hydrologic technicians venture out in the middle of the storms to collect additional data, and to ensure that the data are accurate by calibrating the data collection systems during high-water flows. A series of frontal storms are bringing rain and snow to the entire state of California this month. The USGS CAWSC has deployed 25 teams from six of its eight field offices to conduct flow measurements. Streamgages can be damaged from fast moving flood and debris flows, but so far, no damage to instruments has occurred from these events. Streamflow measurements are coordinated ahead of storms with federal, state and local cooperating agencies, so USGS crews will be out again all winter and spring as rainfall continues, snow begins to melt, rivers rise, and flooding occurs.

Image: High Water Measurements at USGS Site 11059300
Jon Newby and Travis Kordiak, USGS Hydrographers from the California Water Science Center Redlands Field Office, take high-water measurements at USGS streamgaging site 11059300. The site is on the Santa Ana River in San Bernardino, California, and is one of more than 500 streamgage locations in California.Location: USGS site 11059300 Santa Ana River at E Street near San Bernardino, CAPublic domain


As part of its natural hazards mission, the USGS studies landslides, including what triggers them, and what damage they can cause.

Landslides are often triggered by heavy rainfall. The combination of steep slopes, weak rocks, and intense winter rainstorms make the San Francisco Bay Area uplands an ideal setting for landslides. During the drenching winter of 1997-98, the strongest El Niño of the 20th century triggered a range of landslides in the Bay Area from deadly debris flows to destructive deep-seated slides.

A deep-seated landslide in the Santa Cruz mountains
A deep-seated landslide in the Santa Cruz mountains of the San Francisco Bay area during April 1998. Multiple scarps (sliding planes) are visible with the trees tilted backwards and the house tilted forwards. (Photo by Robert Schuster, USGS)

The amount of rainfall during a given time and the degree of soil saturation can greatly affect whether the ground will fail and slide down a steep slope. During an El Niño year, with greater than average rainfall, steep slopes can be particularly susceptible to landslides as the ground becomes wetter throughout the rainy season. A continuous series of El Niño storms over a few months can saturate the soil, bringing a slope to the point of failure. This, along with additional high-intensity rainfall can trigger landslides. The USGS conducts active research on identifying the triggering mechanisms and hazards associated with landslides.

Image: Installing Landslide Study Instruments
USGS landslide scientist Brian Collins installing monitoring equipment on a steep hillside in the San Francisco Bay Area. The instrumentation includes subsurface soil moisture and water pressure sensors, and a rain gage. The USGS is monitoring soil moisture and saturation to understand how, where, and when rainfall leads to the triggering conditions for landslides.Location: San Bruno Mountain, San Mateo County, CaliforniaPublic domain

In northern and central California, landslide research efforts have focused on the San Francisco Bay area, and a number of research products are available that showcase the landslide effects of previous large winter storms to the region, including some related to El Niño events.

Like the northern part of the state, southern California is well known to be susceptible to landslides, and particularly those regions that receive significant rainfall. Some, but not all, of the major winter storms that have caused landslide fatalities and property damage in southern California have occurred duringEl Niño conditions.

A particularly dangerous type of landslide, debris flows, frequently follow in areas burned by wildfire.

Debris Flows after Wildfire

Areas recently burned by wildfires are very susceptible to flash floods and debris flows during rainstorms. If an El Niño weather pattern brings more rainfall than usual, conditions can be ripe for disaster in the burned areas, and for downstream communities.

California wildfires are common in the late summer and fall when the conditions are hot, dry, and windy. When the rainy season starts in late fall and through the winter, USGS scientists pay close attention to those areas that have been burned, monitoring conditions that can become ripe for triggering potentially deadly debris flows. An El Niño weather pattern such as we are seeing this year, can exacerbate the situation significantly. More frequent storms can create more runoff and dangerous debris flows in burned areas.

In 2014, the Silverado Fire burned approximately 4 square km (1.5 square miles) in Orange County, California. After the fire, the USGS installed an automated rain-triggered camera to monitor post-wildfire flooding and debris flow at the outlet of a small basin within the burn area. This video shows the initial surge and peak flow triggered by an intense rainstorm on July 19, 2015.

Coastal Hazards

Coastal erosion is not uncommon during large winter storms in California, especially when the storms coincide with high tides. With El Niño bringing more storms and more large waves than typical during the season, the potential for serious beach erosion and cliff retreat is possible. Retreating coastal cliffs cause the loss of structures and property. During the 20th century, about 75 percent of coastal erosion and storm damage in central California occurred during El Niño years.

Image: Severe Coastal Erosion During an El Niño Storm
Severe coastal bluff erosion, along the southern end of Ocean Beach, San Francisco, California. This storm damage occurred during the 2009-2010 El Niño, which, on average, eroded the shoreline 55 meters that winter.Public domain

As part of its Coastal and Marine Geology Program, USGS took a series of aerial photographs to assess coastal erosion from severe storms. USGS acquired baseline coverage of over 1000 km (620 miles) of coastline from the west coast of the U.S. in October 1997, in anticipation of storms generated by the El-Niño warming of the Pacific Ocean. A follow-up mission was completed in April 1998 after the storm season. Comparison photos before and after El Niño storms in the winter of 1997-98 along the coast from Washington to southern California are available online.

Three images showing the effects of coastal erosion
Coastal cliffs are subject to wave action as well as precipitation-induced seepage. These examples from both northern and southern California illustrate several different styles of failure that can occur both at the bottom of cliffs (from burial) and at the tops of cliffs (from falling over).

Storm damage

If the expected upsurge of severe El Niño events causes an increase in storm events, it can lead to extreme coastal flooding and erosion in populated regions across the Pacific Ocean,

Advancing the scientific knowledge of the impacts of El Niño and understanding the effects of severe storms fueled by it helps coastal managers prepare communities for the expected erosion and flooding associated with this climate cycle. Whether vacation destinations in California, or isolated atoll islands in the Pacific Ocean, most coastal areas will feel the effects of El Niño.

Image: Coral reefs, Kwajalein Atoll, Republic of the Marshall Islands
Aerial photograph of Kwajalein Atoll showing its low-lying islands and coral reefs.Public domain

Coastal erosion on the U.S. West Coast spiked in the winter of 2009-10. Getting a better understanding of how the 2009-10 conditions tore away and reshaped shorelines will help coastal experts better predict future changes that may be in store for the Pacific coast. The stormy conditions of the 2009-10 El Niño winter eroded the beaches to often-unprecedented levels at sites throughout California and vulnerable sites in the Pacific Northwest. In California, for example, winter wave energy was 20 percent above average for the years dating back to 1997, resulting in shoreline erosion that exceeded the average by 36 percent. Among the most severe erosion was at Ocean Beach in San Francisco where the winter shoreline retreated 184 feet, 75 percent more than in a typical winter. The erosion resulted in the collapse of one lane of a major roadway and led to a $5 million emergency remediation project.

Image: Coastal Erosion in San Francisco
Winter storm erosion of coastal bluffs at Ocean Beach, San Francisco, January 2010.Public domain
A highway guardrail's posts hang exposed over a tall, eroded, dropoff to the ocean water.
Severe bluff erosion, along the southern end of Ocean Beach, San Francisco, California, including damage to the guard rail of the Great Highway (Calif. Hwy.1). The severe winter erosion led to lane closures of the highway and an emergency, $5 million revetment along the base of this bluff. This storm damage occurred during the 2009-2010 El Niño, which, on average, eroded the shoreline 55 meters that winter.Public domain

In the U.S. Pacific Northwest, the impacts were moderate in 2009-10, but the southerly shift in storm tracks, typical of El Niño winters, resulted in severe local wave impacts to the north-of-harbor mouths and tidal inlets. For example, north of the entrance to Willapa Bay along the Washington coast, 345 feet of shoreline erosion during the winter of 2009-10 destroyed a road.

USGS scientists are using all the tools at their disposal to explore the many facets of this complex El Niño phenomenon including the Coastal Storm Modeling System (CoSMoS) to project extreme-storm flooding and erosion along the California coast. State and local agencies use these modeled projections to plan for coastal impacts from El Niño storms.

Fort Point Sea-Level Record
Sea-level measurements collected at Fort Point in San Francisco since before 1900 form the longest continuous sea-level record for any site on the west coast of North America. This record was analyzed by USGS scientists, who found that four major factors influence sea level at Fort Point—daily tides, annual sea-level cycles, a long-term trend of slowly rising sea level (red line).

Global climate change and sea-level rise

The impact of El Niño and other storms is not presently included in most studies on future coastal vulnerability in a changing climate, which look primarily at long-term sea-level rise.New research suggests the predicted El Niño increase in storms will exacerbate coastal erosion. A wet winter with frequent storms can flood low-lying or coastal areas, especially if the average sea level is higher than in the past.

Historical Studies of past El Niño Events

El Niño is not a new phenomenon, it is with us now, and it will surely visit us again in the future. Learning from studies of past El Niño events teaches us to prepare for future occurrences, making all communities more resilient to Nature’s cycles.

Landslides, Floods, and Marine Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region, California

1982-83 El Niño Coastal Erosion: San Mateo County, California

Floods in California, January 1997

Coastal Change Hazards: 1997-98 El Niño in San Mateo County, California

Comparison photos before and after El Niño storms in the winter of 1997-98 along the coast from Washington to southern California.

Coastal Impacts of an El Niño Season, 1998 (poster)

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