Scientists from the U.S. Geological Survey (USGS) are finding evidence of a much wetter past in the deserts of the American 

Southwest using a most unlikely source—wetlands. Wetlands form in arid environments where water tables approach or breach the ground 

surface. Often thought of as stagnant and unchanging, new evidence suggests that springs and wetlands responded dynamically to past 

episodes of abrupt climate change. Multiple cycles of deposition, erosion, and soil formation show that wetlands in the southwestern United 

States expanded and contracted many times during the past 35,000 years or so, before disappearing altogether as the last glacial period 

came to a close. USGS scientists are now studying the deposits to determine how closely conditions in the desert were tied to regional and 

global climate patterns in the past, and what it might mean for the fragile ecosystems in light of anticipated climate change in the future.

Desert Wetlands—Archives of a Wetter Past

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3077

December 2015 Printed on recycled paper ISSN 2327-6916 (print) ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153077

Wetlands in the Geologic Record

lakes and offer several advantages. First, 

they provide unequivocal evidence of 

past water-table levels. In many instances, 

the elevation of past discharge can be 

identified to within a few centimeters. 

Second, they are often exposed over fairly 

large areas, which affords researchers 

the opportunity to examine and interpret 

complex sedimentological features 

that can be missed in sediment cores. 

Third, they can be dated by multiple 

techniques and, therefore, hydrologic and 

climatic conditions recorded by wetlands 

can be tied directly to other climate 

records, such as ice cores, marine and 

lake sediments, and speleothems (cave 

deposits). Finally, they are relatively 

common in the world’s arid lands and can 

be used to reconstruct past conditions on a 

variety of spatial and temporal scales.

Wetland deposits are also important 

archeological and paleontological archives. 

Active wetlands provide reliable sources of 

fresh water in an otherwise arid landscape 

and thus act as focal points for human 

and faunal activities. The remains of large 

vertebrates are frequently preserved in 

wetland deposits, which allows researchers to examine how faunal communities 

responded to past episodes of climate 

change in arid environments, as well 

as to investigate potential ties between 

megafauna extinctions, changes in past 

environmental conditions, and the arrival 

of humans.

Figure 1. In southern Nevada, buff-colored deposits sit against the backdrop of the Las Vegas 

Range just outside the city limits of Las Vegas. Once thought to be remnants of a large lake, 

these deposits actually record the presence of extensive wetlands that were home to an 

array of Pleistocene megafauna. They also hold valuable clues to understanding how desert 

ecosystems responded to climate change in the past. (Photograph courtesy of Eric Scott.)

As continental ice sheets waxed 

and waned during the late Pleistocene, 

global reorganizations of the atmosphere, 

biosphere, and hydrosphere resulted in 

dramatic physical changes in the deserts of 

the American Southwest. Plant communities were often displaced 1,000 meters 

or more downslope, perennial lakes 

were present where only dry playas exist 

today, and large animals (megafauna) 

lived in areas that are now too hot and 

dry to support them. The cooler, wetter 

conditions of the late Pleistocene also 

supported higher groundwater levels in 

the desert, which fed extensive networks 

of marshes, wet meadows, seeps, and 

flowing springs in many low-lying valleys 

and basins.

In wetland settings, eolian, alluvial, 

and fluvial sediments are trapped by a 

combination of wet ground conditions 

and dense plant cover. Over time, these 

sediments build up and are ultimately 

preserved in the geologic record as a 

unique combination of clastic sediments, 

chemical precipitates, and organic matter 

that are collectively called “paleowetland 

deposits” (fig. 1). Paleowetland deposits 

contain information on the timing and 

magnitude of past changes in watertable levels and, therefore, can be 

important sources of hydrologic and 

climatic information.

Traditionally, geologists have 

relied heavily on lake deposits to 

reconstruct the magnitude and timing of 

environmental changes that took place in 

the American Southwest. Wetland deposits 

contain complementary information to 
Ecosystem Response to Climate Change in the Las Vegas Valley

Dr. C. Vance Haynes, Jr.

and the birth of desert wetland research

firm geologic context, and to understand 

how the hydrologic systems responded to 

climate change in the past.

The results of the investigations 

show that wetlands in the Las Vegas 

Valley expanded and contracted many 

times in response to changing climate 

conditions during the late Pleistocene 

(fig. 2). Cessation of groundwater 

discharge associated with rapid warming 

events resulted in the collapse of entire 

wetland systems at multiple times 

during the late Quaternary. Drought-like 

conditions in the valley, as recorded by 

widespread erosion and the formation 

of desert soils, typically lasted for a few 

centuries, which illustrates the potential 

vulnerability of these fragile ecosystems 

to climate change.

USGS scientists are currently 

investigating wetland deposits elsewhere 

in the southern Great Basin and Mojave 

Deserts to determine how water tables 

responded to past changes in climate on 

a regional scale. Information yet to be 

discovered could help answer critical 

questions about how these ecosystems 

might behave in the future. The results 

may also have profound implications 

for those managing active springs and 

wetlands today, as they are often home to 

threatened and endangered species living 

in the deserts of the American Southwest.

—By Jeffrey S. Pigati, Kathleen B. Springer, 

Craig R. Manker

Renowned for his pioneering 

work on the peopling of North America, 

Dr. C. Vance Haynes, Jr., is also the 

godfather of desert wetland research in 

the southwestern United States. In 1962, 

he began studies of wetland deposits 

at the Tule Springs archeological site 

located in the Upper Las Vegas Wash 

to understand what drew the ancient 

people to the area, how they survived in 

southern Nevada’s arid conditions, and 

whether they coexisted with Pleistocene 

megafauna. Nearly a half-century later, 

USGS scientists continue to refine the 

geologic and chronologic frameworks 

that Dr. Haynes established.

For more information contact:

U.S. Geological Survey

Geosciences and Environmental 

Change Science Center

http://gec.cr.usgs.gov/

Paleohydrology of 

Desert Wetlands project

This fact sheet is available online at 

http://dx.doi.org/10.3133/fs20153077

Dr. C. Vance Haynes, Jr., at the Tule 

Springs archeological site near Las Vegas. 

(Photograph courtesy of San Bernardino 

County Museum.)

Figure 2. Types of groundwater discharge 

in the Las Vegas Valley compared to isotopic 

values from the Greenland ice core record 

(higher isotopic values to the right represent 

warmer conditions; lower values represent 

cooler conditions). Wetland expansion and 

contraction in the Las Vegas Valley marched 

in virtual lockstep with short-lived, northern 

hemispheric climate oscillations, including 

Dansgaard-Oeschger (D-O) cycles, the 

Bølling (B) and Allerød (A) warm periods, 

and the Younger Dryas cold event (YD).

Today, a harsh desert landscape 

surrounds the city of Las Vegas. Boasting 

extreme temperatures and dry weather for 

much of the year, Las Vegas was founded 

in the mid-19th century near a small set 

of springs and marshes that provided 

a continuous source of water to early 

residents. In prehistoric times, the marshes, 

or “meadows” for which the city is named, 

were far more extensive and covered 

much of the valley. The marshes were also 

home to some of North America’s earliest 

human inhabitants, as well as a diverse

assemblage of Pleistocene megafauna,

including mammoth, sloth, bison, camel,

horse, and even American lion and sabretoothed cat.

Early scientific work in Las Vegas 

Valley focused largely on the archeological sites that are prevalent in the area. 

More recently, a collaborative effort 

between the USGS and San Bernardino 

County Museum (SBCM), with funding 

by the Bureau of Land Management, has 

focused on investigating paleowetland 

deposits in the Upper Las Vegas Wash, 

located just outside the city limits. 

There, SBCM scientists discovered and 

excavated hundreds of fossil sites in 

what is now Tule Springs Fossil Beds 

National Monument. In support of these 

efforts, USGS scientists have helped 

refine the stratigraphy and chronology 

of the Las Vegas Valley paleowetland 

deposits in order to place the fossils in a 

EXPLANATION

Streams Ponds Marshes

B (DO-1)

A

D-O 2

D-O 3

D-O 4

D-O 5

D-O 6

D-O 7

YD

Lithologic breaks

or erosion

Aridisol

Widespread

erosion

Oxidized sediment

Aridisol

Aridisol

Erosion

Widespread

erosion

Aridisol

Aridisol

–50 –45 –40 –35 –30

GICC05 δ18O (‰)

Age (thousands of years before present)

5

10

15

20

25

30

35