Unified Interior Regions

California

The Southwest Region includes California, Nevada, and Arizona. The Regional Office, headquartered in Sacramento, provides Center oversight and support, facilitates internal and external collaborations, and works to further USGS strategic science directions.

States L2 Landing Page Tabs

Filter Total Items: 389
Date published: October 30, 2017
Status: Active

Wetland Restoration in the San Francisco Bay Delta and Pacific Northwest

Estuaries and healthy coastal habitats are among the most productive ecosystems on Earth. They provide a variety of benefits, including habitat and food for fish and wildlife, flood and erosion protection, improved water quality, increased carbon sequestration, as well as beautiful scenery and opportunities for recreation.  Along the U.S. Pacific Coast, both the San Francisco Bay estuary and...

Date published: October 30, 2017
Status: Active

Contaminants in Waterbirds and Effects on Avian Reproduction

California’s Central Valley and San Francisco Bay Estuary have a long history of mercury contamination from past mercury mining and gold extraction. Waterbirds are particularly susceptible to mercury because of their use of wetland habitats where methylmercury (the most toxic and biologically available form) is produced and relatively low methylmercury exposure can reduce reproductive success...

Contacts: Josh T Ackerman
Date published: October 30, 2017
Status: Active

California Sea Otter Surveys and Research

WERC collaborates with other research scientists to conduct annual population surveys of the southern sea otter -- a federally listed threatened species. In coordination with the California Department of Fish and Game and other institutions, ongoing surveys and research continues to inform the southern sea otter recovery plan for the U.S. Fish and Wildlife Service, and contributes to our...

Date published: October 30, 2017
Status: Active

Mercury Bioaccumulation in Wetlands

Wetlands provide numerous ecosystem services, but also can be sources of methylmercury production and export. Click the next tab to learn how WERC's Dr. Josh Ackerman is evaluating the ecological factors that drive contaminant bioaccumulation in wetland-dependent fish and wildlife.

Contacts: Josh T Ackerman
Date published: October 30, 2017
Status: Active

Population Biology and Behavior of Sea Otters

WERC's sea otter researchers are developing and utilizing a variety of methodological and analytical tools to understand the causes of biological and ecological trends in sea otter populations, and to predict the ecological consequences of management practices on these populations and their ecosystems.

Date published: October 30, 2017
Status: Active

Pacific Nearshore Project

Sea otters are crucial indicators of the health of our nearshore waters and coastal resources, from kelp forests to fisheries. What clues does the sea otter's decline hold for our knowledge of ecosystem and global change? WERC's sea otter team and U.S. and Canadian researchers have teamed together to investigate. 

Relevance to USGS Missions:

This research project has direct...

Date published: October 30, 2017
Status: Active

Waterfowl Ecology in California and the Pacific Flyway

The Suisun Marsh and Central Valley in California offer some of the world’s most important wetland habitats for waterfowl in the Pacific Flyway. Mike Casazza and USGS WERC biologists are providing the science to support and evaluate waterfowl populations and habitat management in North America.

Contacts: Michael Casazza
Date published: October 30, 2017
Status: Active

Modeling Sea-Level Rise in San Francisco Bay Estuary

With sea level rise, how will the coastal habitats of the San Francisco Bay Estuary change over the next 100 years? Mapping and modeling studies by Dr. Karen Thorne, WERC scientists, and partners have produced scenarios for this important coastal ecosystem.

Date published: October 30, 2017
Status: Active

Ecology and Population Dynamics of Ridgway's Rails along the West Coast of the U.S.

The Ridgway’s rail is a federal and state listed endangered species that occurs in wetlands along the Pacific Coast and from the Lower Colorado River drainage to southern Baja California. Three subspecies of Ridgway’s rail are found within the United States: the California Ridgway’s Rail, Yuma Ridgway’s rail, and Light-footed Ridgway’s rail.  All three subspecies have declined since 1900 as a...

Contacts: Michael Casazza
Date published: October 30, 2017
Status: Active

Wildland Fire Science in Forests and Deserts

Fuel conditions and fire regimes in western forests and deserts have been altered due to past land management, biological invasions, and recent extreme weather events and climate shifts. These changes have created extreme fire risk to local and regional communities, threatening their economic health related to wildland recreation, forest production, livestock operations, and other uses of...

Contacts: Matthew Brooks
Date published: October 30, 2017
Status: Active

Breeding and Wintering Ecology of Waterfowl

Western U.S. wetlands provide critical habitat for wintering and breeding waterfowl in California. WERC's Dr. Josh Ackerman is working toward collecting data to understand factors influencing duck nest success, to improve and restore breeding habitat for resident duck populations in California, and understand composition of predator communities. To learn more about how USGS WERC is...

Contacts: Josh T Ackerman
Date published: October 30, 2017
Status: Active

Coastal Ecosystem Response to Sea-level Rise

USGS WERC’s Dr. Karen Thorne, her team of reseachers, and her partners are currently taking a local site network approach to describe current and future conditions and projected responses of coastal ecosystems to sea-level rise and other stressors. The Coastal Ecosystem Response to Climate Change (CERCC) program’s goal is to understand how ecosystems vary in their ability to keep up with sea-...

Filter Total Items: 517
Date published: January 19, 2018

Oblique Aerial Photography Viewer

Obique photos offer a unique perspective of the coast. Features such as beach erosion or accretion, dune erosion and overwash can all be clearly characterized in this imagery. It also documents coastal infrastructure, as well as the damage that infrastructure may incur as the result of an impacting hurricane. 

Date published: January 1, 2018

Magnetotelluric Data from the San Andreas Fault, Parkfield CA, 1990

The U.S. Geological Survey (USGS) Geology, Geophysics and Geochemistry Science Center (GGGSC) collaborated with the USGS Data at Risk (DaR) team to preserve and release a subset of magnetotelluric data from the San Andreas Fault in Parkfield, California. The San Andreas Fault data were collected by the Branch of Geophysics, a precursor to the now GGGSC, between 1989 and 1994.

Date published: January 1, 2018

Geochemical and geophysical data for wells in the Fruitvale oil and gas field

The U.S. Geological Survey (USGS) in cooperation with the California State Water Resources Control Board compiled and analyzed data for the purpose of mapping groundwater salinity in selected oil and gas fields in California. The data for the Fruitvale and Rosedale Ranch oil fields include well construction data, digitized borehole geophysical data, geochemical analyses of water samples f

Date published: January 1, 2018

River Channel Survey Data, Redwood Creek, California, 1953-2013

Dr. Richard Janda of the USGS began a channel monitoring program in Redwood Creek in northern coastal California in 1973. The USGS continued this work through 2013, when the Research Geologist, Dr. Mary Madej retired. This effort produced 40 years of channel change data in rivers that were disrupted by severe erosion following timber harvest of old-growth redwood forests, a portion of the p...

Date published: January 1, 2018

CoSMoS (Coastal Storm Modeling System) Southern California v3.0 Phase 2 wave-hazard projections: 1-year storm in Channel Islands

Model-derived significant wave height (in meters) for the given storm condition and sea-level rise (SLR) scenario.
The Coastal Storm Modeling System (CoSMoS) makes detailed predictions (meter-scale) over large geographic scales (100s of kilometers) of storm-induced coastal flooding and erosion for both current and future sea-level rise (SLR) scenarios. CoSMoS v3.0 for So

Date published: January 1, 2018

Water well data used for preliminary regional groundwater salinity mapping near selected oil fields in central and southern California

This digital dataset is comprised of three separate data files that contain total dissolved solids, well construction, and well identifying information for 3,546 water wells used to map salinity in and around 31 southern and central California oil fields. Salinity mapping was done for 27 fields located in the southern San Joaquin Valley of Kern County (North Belridge, South Belridge,

Date published: January 1, 2018

Grain size, mineralogic, and trace-element data from field samples near Hinkley, California

This data release is part of a study examining the occurrence of chromium and natural and anthropogenic hexavalent Chromium, Cr(VI) in groundwater. Data will be used to estimate naturally-occurring background Cr(VI) concentrations upgradient, near the plume margins, and downgradient from a mapped Cr(VI) contamination plume near Hinkley, CA (Izbicki and Groover, 2016).

Date published: January 1, 2018

Study Boundary and Grid Cells for the North and South American, Cosumnes, and Mokelumne River Watersheds Shallow Aquifer Study Unit

The U.S. Geological Survey collected groundwater samples from 67 domestic wells and 1 spring used for domestic supply in Placer, El Dorado, Amador, and Calaveras Counties, California in 2016-2017. The sites were sampled for the Mokelumne, Cosumnes, and American River Watersheds (MCAW) Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambi

Date published: January 1, 2018

High-Resolution Aeromagnetic Survey of Mountain Pass, California

A detailed aeromagnetic survey of Mountain Pass, California was flown by CGG Canada Services Ltd. (CGG) during November and December, 2016. The high-resolution helicopter survey was flown at a flightline spacing of 100 and 200 m, a flightline azimuthal direction of 70 degrees, a nominal flightline elevation above ground of 70 m, and consists of about 1,814 line-kilometers. Tie l

Date published: January 1, 2018

CoSMoS (Coastal Storm Modeling System) Southern California v3.0 Phase 2 wave-hazard projections: average conditions in Channel Islands

Model-derived significant wave height (in meters) for the given storm condition and sea-level rise (SLR) scenario.
The Coastal Storm Modeling System (CoSMoS) makes detailed predictions (meter-scale) over large geographic scales (100s of kilometers) of storm-induced coastal flooding and erosion for both current and future sea-level rise (SLR) scenarios. CoSMoS v3.0 for So

Date published: January 1, 2018

Recent historical and projected (years 2006–99) areas (km2) of managed, flooded habitats used by waterbirds overwintering in Central Valley, California basins for 17 climate, urbanization, and water management scenarios.

Matchett and Fleskes (2018) evaluated availability of wetland and agricultural habitats used by waterbirds overwintering in the Central Valley of California under 17 scenarios of projected climate, urbanization, and water supply management (for more information about scenarios see scenario description and Table 1 in Matchett and Fleskes [2018]). Central Valley waterbird habitats investigated...

Filter Total Items: 354
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Year Published: 1999

Map showing locations of damaging landslides in San Francisco City and County, California, resulting from 1997-98 El Nino rainstorms

Heavy rainfall associated with a strong El Nino caused over $150 million in landslide damage in the 10-county San Francisco Bay region during the winter and spring of 1998. A team of USGS scientists collected information on landslide locations and damage costs. About $4 million in damages were assessed in San Francisco City and County.

Hillhouse, John W.; Godt, Jonathan W.
Map showing locations of damaging landslides in San Francisco City and County, California, resulting from 1997-98 El Nino rainstorms; 1999; MF; 2325-G; Hillhouse, John W.; Godt, Jonathan W.

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Year Published: 1999

Map showing locations of damaging landslides in San Mateo County, California, resulting from 1997-98 El Nino rainstorms

Heavy rainfall associated with a strong El Nino caused over $150 million in landslide damage in the 10-county San Francisco Bay region during the winter and spring of 1998. A team of USGS scientists collected information on landslide locations and damage costs. About $55 million in damages were assessed in San Mateo County. The only fatality...

Jayko, Angela S.; De Mouthe, Jean; Lajoie, Kenneth R.; Ramsey, David W.; Godt, Jonathan W.
Map showing locations of damaging landslides in San Mateo County, California, resulting from 1997-98 El Nino rainstorms; 1999; MF; 2325-H; Jayko, Angela S.; De Mouthe, Jean; Lajoie, Kenneth R.; Ramsey, David W.; Godt, Jonathan W.

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Year Published: 1999

Map showing locations of damaging landslides in Santa Clara County, California, resulting from 1997-98 El Nino rainstorms

Heavy rainfall associated with a strong El Nino caused over $150 million in landslide damage in the 10-county San Francisco Bay region during the winter and spring of 1998. A team of USGS scientists collected information on landslide locations and damage costs. About $7.6 million in damages were assessed in Santa Clara County.

Ellis, William L.; Harp, Edwin L.; Arnal, Caroline H.; Godt, Jonathan W.
Map showing locations of damaging landslides in Santa Clara County, California, resulting from 1997-98 El Nino rainstorms; 1999; MF; 2325-J; Ellis, William L.; Harp, Edwin L.; Arnal, Caroline H.; Godt, Jonathan W.

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Year Published: 1999

Map showing locations of damaging landslides in Santa Cruz County, California, resulting from 1997-98 El Nino rainstorms

Heavy rainfall associated with a strong El Nino caused over $150 million in landslide damage in the 10-county San Francisco Bay region during the winter and spring of 1998. A team of USGS scientists collected information on landslide locations and damage costs. About $14.5 million in damages were assessed in Santa Cruz County.

Baum, Rex L.; Schuster, Robert L.; Godt, Jonathan W.
Map showing locations of damaging landslides in Santa Cruz County, California, resulting from 1997-98 El Nino rainstorms; 1999; MF; 2325-D; Baum, Rex L.; Schuster, Robert L.; Godt, Jonathan W.

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Year Published: 1999

Map showing locations of damaging landslides in Solano County, California, resulting from 1997-98 El Nino rainstorms

Heavy rainfall associated with a strong El Nino caused over $150 million in landslide damage in the 10-county San Francisco Bay region during the winter and spring of 1998. A team of USGS scientists collected information on landslide locations and damage costs. About $13.5 million in damages were assessed in Solano County.

Howell, David G.; Godt, Jonathan W.
Map showing locations of damaging landslides in Solano County, California, resulting from 1997-98 El Nino rainstorms; 1999; MF; 2325-I; Howell, David G.; Godt, Jonathan W.

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Year Published: 1999

Map showing locations of damaging landslides in Sonoma County, California, resulting from 1997-98 El Nino rainstorms

Heavy rainfall associated with a strong El Nino caused over $150 million in landslide damage in the 10-county San Francisco Bay region during the winter and spring of 1998. A team of USGS scientists collected information on landslide locations and damage costs. About $21 million in damages were assessed in Sonoma County.

Ramsey, David W.; Godt, Jonathan W.
Map showing locations of damaging landslides in Sonoma County, California, resulting from 1997-98 El Nino rainstorms; 1999; MF; 2325-F; Ramsey, David W.; Godt, Jonathan W.

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Year Published: 1998

Chapter A. The Loma Prieta, California, Earthquake of October 17, 1989 - Lifelines

To the general public who had their televisions tuned to watch the World Series, the 1989 Loma Prieta earthquake was a lifelines earthquake. It was the images seen around the world of the collapsed Cypress Street viaduct, with the frantic and heroic efforts to pull survivors from the structure that was billowing smoke; the collapsed section of the...

Schiff, Anshel J.
Chapter A. The Loma Prieta, California, Earthquake of October 17, 1989 - Lifelines; 1998; PP; 1552-A; The Loma Prieta, California, Earthquake of October 17, 1989: Performance of the Built Environment; Edited by Schiff, Anshel J.

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Year Published: 1998

Chapter D. The Loma Prieta, California, Earthquake of October 17, 1989 - Earth Structures and Engineering Characterization of Ground Motion

This chapter contains two papers that summarize the performance of engineered earth structures, dams and stabilized excavations in soil, and two papers that characterize for engineering purposes the attenuation of ground motion with distance during the Loma Prieta earthquake. Documenting the field performance of engineered structures and...

Holzer, Thomas L.
Chapter D. The Loma Prieta, California, Earthquake of October 17, 1989 - Earth Structures and Engineering Characterization of Ground Motion; 1998; PP; 1552-D; The Loma Prieta, California, Earthquake of October 17, 1989: Performance of the Built Environment; Edited by Holzer, Thomas L.

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Year Published: 1998

Chapter D. The Loma Prieta, California, Earthquake of October 17, 1989 - Recovery, Mitigation, and Reconstruction

The papers in this chapter reflect the broad spectrum of issues that arise following a major damaging urban earthquake-the regional economic consequences, rehousing problems, reconstruction strategies and policies, and opportunities for mitigation before the next major seismic event. While some of these papers deal with structural or physical...

Nigg, Joanne M.
Chapter D. The Loma Prieta, California, Earthquake of October 17, 1989 - Recovery, Mitigation, and Reconstruction; 1998; PP; 1553-D; The Loma Prieta, California, Earthquake of October 17, 1989: Societal Response; Edited by Nigg, Joanne M.

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Year Published: 1998

Chapter B. The Loma Prieta, California, Earthquake of October 17, 1989 - Forecasts

The magnitude (Mw) 6.9 Loma Prieta earthquake struck the San Francisco Bay region of central California at 5:04 p.m. P.d.t. on October 17, 1989, killing 62 people and generating billions of dollars in property damage. Scientists were not surprised by the occurrence of a destructive earthquake in this region and had, in fact, been attempting to...

Harris, Ruth A.
Chapter B. The Loma Prieta, California, Earthquake of October 17, 1989 - Forecasts; 1998; PP; 1550-B; The Loma Prieta, California, Earthquake of October 17, 1989: Earthquake Occurrence; Edited by Harris, Ruth A.

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Year Published: 1998

Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 - Building Structures

Several approaches are used to assess the performance of the built environment following an earthquake -- preliminary damage surveys conducted by professionals, detailed studies of individual structures, and statistical analyses of groups of structures. Reports of damage that are issued by many organizations immediately following an earthquake...

Çelebi, Mehmet
Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 - Building Structures; 1998; PP; 1552-C; The Loma Prieta, California, Earthquake of October 17, 1989: Performance of the Built Environment; Edited by Celebi, Mehmet

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Year Published: 1998

Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 - Landslides

Central California, in the vicinity of San Francisco and Monterey Bays, has a history of fatal and damaging landslides, triggered by heavy rainfall, coastal and stream erosion, construction activity, and earthquakes. The great 1906 San Francisco earthquake (MS=8.2-8.3) generated more than 10,000 landslides throughout an area of 32,000 km2; these...

Keefer, David K.
Chapter C. The Loma Prieta, California, Earthquake of October 17, 1989 - Landslides; 1998; PP; 1551-C; The Loma Prieta, California, Earthquake of October 17, 1989: Strong Ground Motion and Ground Failure; Edited by Keefer, David K.

Filter Total Items: 981
erry Hatcher with the system he designed to record the precise time and geographic location of each air photo Van Wagenen shoots
December 31, 2017

Scientist with the system he designed to record precise photo data

Gerry Hatcher with the system he designed to record the precise time and geographic location of each air photo Van Wagenen shoots.

Medicine Lake, California simplified hazards map showing potential ...
December 31, 2017

Medicine Lake, CA simplified hazards map

Medicine Lake, California simplified hazards map showing potential impact area for ground-based hazards during a volcanic event.

man in yellow shirt and camo coveralls standing in thigh-high water with instrument.
December 31, 2017

Measuring streamflow in the Sisquoc River near Garey, California.

USGS technician measuring streamflow in the Sisquoc River near Garey, California, upstream of the Santa Maria Mesa Road Bridge.

USGS scientist collecting samples of aquatic species from the Pike River, Wis., for later analysis of mercury contamination.
December 31, 2017

Collecting samples of aquatic species from the Pike River, Wisconsin

Microbial Biogeochemistry Laboratory — Menlo Park, California. USGS scientist collecting samples of aquatic species from the Pike River, Wis., for later analysis of mercury contamination.

scientific instruments and platform next to river surrounded by snow
December 31, 2017

USGS Streamgaging on the Pit River operates year round.

 USGS Streamgaging equipment and cableway on the Pit River near Canby, California operates year round.

Big Sur coast. Red squares mark some of the sites damaged by 2016–17 winter storms, including Mud Creek and Paul’s Slide areas
December 31, 2017

Map of Big Sur coast with labels

Big Sur coast. Red squares mark some of the sites damaged by 2016–17 winter storms, including Mud Creek and Paul’s Slide areas.

Two technicians in a cablecar over a river, with an instrument hanging below them
December 31, 2017

Lowering an acoustic Doppler current profiler from a cable way.

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. 

 model grid with four rows and four columns showing A, types of cascading hydrologic response units (HRUs) and B, land-surface a
December 31, 2017

Model Grids with Cascading Hydrologic Response Units

Hypothetical model grid showing A, types of cascading hydrologic response units and B, land-surface altitudes and cascading flow directions.

Air photo of Paul's slide (left); photo of repair equipment/construction (right)
December 31, 2017

Air photo, Paul's slide (left); repair equipment/construction (right)

Paul’s Slide. Left: Air photo taken May 27, 2017, one of many that Beth Haddon will analyze with structure-from-motion software to measure changes in ground elevation. Right: Caltrans employees and contractors use an excavator and bulldozer to remove Paul’s Slide debris from California State Highway 1. Note recent construction of concrete and a line of grouted reinforcing

...
two men in camo-patterned overalls standing next to a boat on the shore of a river with an instrument on the ground.
December 31, 2017

Technicians prepare to board a boat to measure water flow.

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.

Cheryl Hapke is now sharing her knowledge of the Big Sur region with Warrick’s team
December 31, 2017

Cheryl Hapke shares her knowledge of the Big Sur region

Cheryl Hapke is now sharing her knowledge of the Big Sur region with Warrick’s team, and she is eager to leap back into studies of Big Sur coastal change.

man in yellow slicker standing in rushing river up to his thighs, holding a stick-like instrument in the water
December 31, 2017

Measuring streamflow in fast moving floodwater.

USGS hydrographer measuring streamflow using a handheld Acoustic Doppler Velocimeter in fast moving floodwater Cajon Creek near Keenbrook, California. 

Filter Total Items: 932
USGS science for a changing world logo
June 10, 2014

While most of the Coachella Valley was relatively stable, land surfaces declined about nine inches to two feet in some areas of Palm Desert, Indian Wells, and La Quinta, between 1995 and 2010.

USGS science for a changing world logo
June 10, 2014

While most of the Coachella Valley was relatively stable, land surfaces declined about nine inches to two feet in some areas of Palm Desert, Indian Wells, and La Quinta, between 1995 and 2010.

Flyer for May presentation
May 28, 2014

Can better understanding fish diseases help to create much sought-after cures for human diseases such as Hepatitis C?

Image: Mud and Peat Deposits from Point Reyes Marsh
May 20, 2014

An extensive sedimentary deposit formed by a tsunami in 1946 was recently discovered at Pillar Point Marsh near Half Moon Bay, California. While there were photos and eyewitness accounts of the tsunami and resulting damage at the time, finding the tangible evidence in the geologic record is an important part of assessing the long-term hazard of tsunamis on California coastal communities.

USGS science for a changing world logo
May 19, 2014

Contaminated groundwater found at shallow depths in southern Los Angeles County has the potential to migrate to deeper aquifers, according to a scientific study just published by the U.S. Geological Survey and the Water Replenishment District of Southern California.

USGS science for a changing world logo
May 14, 2014

Nitrate was detected at high concentrations in about 14 percent of untreated groundwater sources used for public water supply in the Livermore, Gilroy-Hollister, and Cuyama valleys of the Southern Coast Ranges, according to an ongoing U.S. Geological Survey study of the state's groundwater quality.

Lidar image showing the upper parts of the landslide that occurred in northwest Washington on March 22, 2014.
May 14, 2014

Want to know how elevation will benefit your state? The USGS National Geospatial Program is advancing the 3D Elevation Program, known as 3DEP, in response to the growing need for high-quality three-dimensional representations of the Nation’s natural and constructed features.

USGS science for a changing world logo
May 7, 2014

Results of a new hydrologic model show a decrease in groundwater levels in response to pumping, which in turn causes a reduction in groundwater discharge to streams, a reduction in groundwater evapotranspiration, and a reduction in groundwater storage in the Santa Rosa Plain watershed.

USGS science for a changing world logo
April 30, 2014

The coastal geology of Simeonof Island, the southeastern-most island in the Shumagin archipelago of the Aleutian Islands, suggests the region has not experienced a great megathrust earthquake in at least the past 3,400 years.

Catastrophic Earthquakes – In a Crowded World Public Lecture Flyer
April 22, 2014

Why have there been so many catastrophic earthquakes at the beginning of the 21stcentury? On April 24th, USGS Research Geologist Thomas Holzer will tell us the history and future of earthquake death tolls, and the urbanization of the planet. Find out what makes modern megacities vulnerable to natural hazards.

USGS science for a changing world logo
April 22, 2014

Inorganic trace elements – fluoride, arsenic, molybdenum and boron – were detected at high concentrations in 42 percent of groundwater used for public supply in the Borrego Valley, and southern desert areas of California, according to a recent study by the U.S. Geological Survey. The USGS evaluated the quality of untreated groundwater for this study – not treated tap water.