69 
 
Floods in the SW USA in August and September 2023 
 
Poplave v JZ delu ZDA v avgustu in septembru 2023 
 
Stanka Šebela
1
,  Barbara Luke
2
 
 
Povzetek 
 
Zaradi orkana Hilary so v JZ delu ZDA razglasili prvo opozorilo v zgodovini. Od 19. do 20. avgusta 
2023 je v južni Nevadi padlo 234 mm padavin, kar je nov rekord in predstavlja podvojitev starega 
rekorda iz leta 1906. Tudi v zveznih državah Montana, Idaho in Oregon so presegli stare rekorde za 
okrog 25 mm. Dne 22. avgusta 2023 je bil zaprt tudi dostop do Grand Canyon South Rim, saj je v 
dveh do treh urah padlo 50 do 76 mm padavin, kar je povzročilo poplavo v kraju Tusayan (Arizona). 
Obilne padavine so se nadaljevale tudi 1. in 2. septembra, ko je tropska nevihta s 50 do 100 mm dežja 
na območju Las Vegasa povzročila poplave. Okrog 20 in 30 km JZ od Las Vegasa so v dveh urah 
zabeležili 81 mm padavin, v 45 minutah kar 60 mm. Zaradi obilnega dežja in naplavin so bili zaprti 
številni narodni parki kot tudi avtocesta med Las Vegasom in Los Angelesom. Monsunsko deževje 
v poznem poletju je v JZ delu ZDA sicer reden pojav, vendar se je v avgustu 2023 intenziteta padavin 
močno povečala, kar pripisujejo klimatskim spremembam in vplivu El Niño.  
 
Ključne besede: poplave, puščava, JZ ZDA. 
 
Abstract 
 
Due to the Hurricane Hilary, the Southwestern United States received its first-ever official 
hurricane warning. From August 19-20, 2023, as much as 234 mm of precipitation fell in southern 
Nevada, which is a new record and represents a doubling of the old record from 1906. The states of 
Montana, Idaho and Oregon also exceeded the old records by about 25 mm. On August 22, 2023, 
access to the Grand Canyon South Rim was closed as 50-76 mm of rain fell in 2-3 hours, causing 
flooding in Tusayan (Arizona). Heavy rainfall continued on September 1-2, when a tropical storm 
with 50-100 mm of rain caused flooding in the Las Vegas area. About 20-30 km SW of Las Vegas, 
81 mm of rain was recorded in two hours, 60 mm in just 45 minutes. Due to heavy rain and debris, 
many national parks were closed as well as the main highway between Las Vegas and Los Angeles. 
Monsoon rains in late summer in the SW part of the USA are a regular phenomenon, but in August 
and September 2023 the intensity of precipitation increased significantly, which is attributed to 
climate change and the influence of El Niño. 
 
Keywords: floods, desert, SW USA. 
 
Introduction 
 
In the framework of the bilateral project ARIS BI-US/22-24-074 "Micro-climatic 
monitoring in tourist caves, comparison of Grand Canyon Caverns with Slovenian caves" in 
the period from 17-29 August 2023 we visited UNLV (University of Nevada, Las Vegas). 
As Hurricane Hilary was approaching the SW part of the USA, we were forced to change 
the planned field trips and adapt to the extraordinary weather conditions. We witnessed 
heavy rain and flooding in the otherwise dry desert southwestern part of the USA. 
 
                                                 
1
 Inštitut za raziskovanje krasa ZRC SAZU, Titov trg 2, Postojna 
2
 Prof.dr. emerita University of Nevada, Las Vegas, ZDA 

70 
 
Hurricane Hilary, which developed from a weather disturbance south of Mexico, entered 
the Pacific Ocean on August 12, 2023 from Central America. It was named Tropical Storm 
Hilary on August 16, 2023, and a Category 4 hurricane on August 17, 2023, with winds of 
up to 236 km/h. The cyclone weakened to a tropical storm over the next few days and made 
landfall 345 km S-SE of San Diego on 20 August 2023 (Wikipedia, 2023a). 
In the states of Arizona, California, Nevada and Utah, 26 million residents were exposed 
to bad weather. On August 18, 2023, the National Hurricane Center (NHC, 2023), which 
was established in 1965, issued its first-ever warning for the desert valleys of Southern 
California. 
Between August 19-20, 2023, the following national parks were fully closed or at least 
partially closed: Joshua Tree National Park, Death Valley National Park, Mojave National 
Preserve, Lake Mead National Recreation Area, and Red Rock Canyon National 
Conservation Area. Portions of I-15, the major highway connecting Los Angeles and Las 
Vegas, were closed to traffic several times during this period due to flooding. 
In the Lee Canyon area (Spring Mountains, Clark County, Nevada, including Mount 
Charleston (3633 m)), 234 mm of precipitation fell during this period, which is a new record 
for Nevada and doubles the old record from 1906 (Prociv, 2023). Also, in the Western US 
states of Montana (old record from 1976), Idaho (old record from 1982) and Oregon (old 
record from 1976) the rainfall exceeded the old records by around 25 mm in August 2023 
(Prociv, 2023). 
A tropical storm like Hilary, which brought torrential rain across four states, is unusual 
and only two similar cases have been recorded in the past. Hurricane Carla produced record 
rainfall in Iowa, Wisconsin, and Michigan in 1961, and Kathleen produced record rainfall in 
California, Oregon, and Idaho in 1976. Hurricane Hilary did not exceed the 1976 rainfall 
record for California, which remains 451 mm (Prociv, 2023). 
Heavy rainfall continued in September as a tropical storm dropped 50-100 mm of rain in 
the Las Vegas area on September 1-2, 2023 (Table 1), flooding many roads especially in the 
E and S parts of the city. At the hydrological station located 8 km SW of Jean (Figure 1), 
which is about 20-30 km SW of Las Vegas, 81 mm of precipitation was recorded in two 
hours, and 60 mm in just 45 minutes (Table 1) (Clemons, 2023). 
 
 
Table 1. Precipitation in southern Nevada for the period August 19-20, 2023 
(after Clemons, 2023). 
Preglednica 1. Količina padavin v južni Nevadi za obdobje 19-20 avgust 2023  
(po Clemons, 2023). 
 
 
mm (48 h) 
Flamingo Wash near Nellis Boulevard, Las Vegas 96 
Grapevine Springs 2, 100 km NW of Las Vegas 85 
Jean, 20-30 km SW of Las Vegas 84 
Las Vegas Wash in Las Vegas 80 
Red Rock Canyon, 30 km W of Las Vegas 66 
Center of Las Vegas 55 
 
Floods in the metropolitan area of Las Vegas (610 m above sea level, Figure 1), which 
has more than 2 million inhabitants, threatened its unhoused people, who are said to number 
at least 1,500 (Signer, 2023). The number of unhoused people is said to have increased in 
recent years due to the opioid crisis and the corona virus pandemic (Signer, 2023). At least 
500 live in more than 100 km of labyrinths and tunnels built under the city to reduce surface 

71 
 
flooding and drain floodwater to the eastern part of the city and into the 19-km long Las 
Vegas Wash. The Wash carries the majority of stormwater along with treated wastewater 
from the Las Vegas Valley through wetlands and finally into Lake Mead, a major reservoir 
on the Colorado River (Torres-Cortez, 2023). The Las Vegas Wash is an urban river, existing 
at its current extent because of the urban population. The drainage works in a systemic 
connection with the wetland in Clark County Wetlands Park, which covers 11.7 km
2
 in the 
eastern part of the Las Vegas Valley (Clark County Wetlands Park, 2023). 
 
 
Figure 1 – The territory of Nevada and Arizona in the SW USA, which were visited in 
August 2023. 1-road, 2-river, 3-mountain top, 4-location described in the paper, 5-city of 
Las Vegas, 6-state border to the two US states (cartography: S. Šebela) 
Slika 1 – Ozemlje Nevade in Arizone na JZ ZDA, ki smo ga obiskali v avgustu 2023. 1-
cesta, 2-reka, 3-vrh gore, 4-lokacija opisana v prispevku, 5-mesto Las Vegas, 6-meja med 
zveznima državama (kartografija: S. Šebela) 
 
 
Las Vegas Springs 
 
The name Las Vegas comes from the Spanish meaning grassy meadows. Originally, 
natural springs flowed in what is today the heart of Las Vegas (Figure 1). The springs 
ensured the survival of the original inhabitants and became instrumental to exploration and 
trade routes across the desert landscape. But by1962 the surface flow stopped due to the 
lowering of the groundwater level due to the pumping of water to supply the growing local 
population.  
Today, the historic springs area is managed as a park which hosts museums, a botanical 
garden, hiking trails and historical interpretations (Springs Preserve, 2023), along with a 
working well field that contributes about 10% of the metropolitan water supply. 
 
 
Red Spring – Calico Basin 
  
One active spring area is the BLM’s (National Conservation Lands, Bureau of Land 
Management) Calico Basin, located just a few miles north of the Red Rock Canyon Visitor 

72 
 
Center. There are three permanent springs in the Calico Basin area: Red Spring (Red Spring 
Picnic Area, 2023), Calico Spring, and Ash Spring. Next to the source of Red Spring, we 
find a grassy swamp (Figure 2A) with rich animal and plant life. This place was also 
favorable for the original inhabitants - the Indigenous People who are said to have lived here 
as early as 6000 years ago, as evidenced by petroglyphs, artifacts and other archaeological 
remains. Heavy rains during the period August 19-20, 2023 increased the flow of the Red 
Spring, which was seen as inundated grass (Figure 2B), and brought a lot of debris onto the 
roads. 
 
 
 
Figure 2 – A - Red Spring Calico Basin, August 21, 2023 (photo: S. Šebela), B - Red 
Spring Calico Basin after heavy rains, August 21, 2023 (photo: S. Bonzanni), C - Red 
Rocks, August 25, 2023 (photo: S. Šebela), D - Red Rocks, August 21, 2023 
(photo: S. Šebela) 
 
Slika 2 – A - Red Spring Calico Basin, 21. avgust 2023 (foto: S. Šebela), B - Red Spring 
Calico Basin po večjem deževju, 21. avgust 2023  (foto: S. Bonzanni), C -  Red Rocks 25. 
avgust 2023 (foto: S. Šebela), D -  Red Rocks, 21. avgust 2023 (foto: S. Šebela) 
 
 
Blue Diamond 
 
We spent quite a few days in Blue Diamond (Figure 1), which is a small independent 
settlement on the way from Las Vegas to the west. It is located in the Red Rock Canyon 
National Conservation Area, which sees more and more visitors every year. The settlement 
of around 280 inhabitants lies at an altitude of 1130 m and is located 41 km SW of Las 
Vegas. It developed in the 1940’s as a settlement to support a nearby gypsum mine that has 
been in operation since 1924 (Blue Diamond Elementary School, 2023), next to Cottonwood 
Spring (also called Ojo de Cayetana or Pearl Spring), located between the springs in the 
Spring Mountains and the springs in Las Vegas. 
Cottonwood Spring was a rest stop and stop along the Old Spanish Trail, which ran for 
about 1,100 km, connecting the area around Santa Fe, New Mexico, and southern California. 
Spanish conquistadors are said to have explored this area in the late 16th century, and 
merchants used this route intensively in the period 1830-1848 (Weimer Purkey, 1994). Later 
the route was called the Mormon Road. 

73 
 
Cottonwood Spring, as well as many other springs, was used by Native Americans as well 
as early settlers of the American West. Today, the spring is used to irrigate a verdant park 
and for drinking water. The water rights are privately owned. 
During periods of heavy rain in Red Rock and the Spring Mountains, the customarily dry 
streambed that separates the village of Blue Diamond from State Road 159, which passes 
through the Red Rock Canyon, fill with water (Figure 2C and D and Figure 3A and B). 
 
 
 
 
Figure 3 – A - Blue Diamond, Arroyo road - flood on July 1, 2016 (photo: S. Šebela), B - 
Blue Diamond, Arroyo road - flood on August 21, 2023 (photo: S. Bonzanni), C - Hoover 
Dam and Lake Mead on August 24, 2023. We can see the decreased water level in the lake. 
The Colorado River constitutes the border between Arizona on the E and Nevada on the W 
(photo: S. Šebela), D - Storm clouds at the Grand Canyon south Rim, August 22, 2023 
(photo: S. Bonzanni) 
 
Slika 3 – A - Blue Diamond, Arroyo road - poplava 1. julija 2016 (foto: S. Šebela), B - 
Blue Diamond, Arroyo road - poplava 21. avgusta 2023 (foto: S. Bonzanni), C - Hoover 
dam in jezero Mead 24. avgusta 2023, opazimo lahko nivo znižanja vode v jezeru, reka 
Kolorado je tudi mejna reka med Arizono na V in Nevado na Z (foto: S. Šebela), D - 
nevihtni oblaki na Grand Canyon south Rim, 22 avgust 2023 (foto: S. Bonzanni) 
 
 
Lake Mead 
 
At the beginning of the 20th century, due to the flooding of the Colorado River, the 
inhabitants of the SW part of the United States began to build a massive dam on the river in 
order to contain the floods, obtain electricity, and manage water for drinking and irrigation. 
At least 3,500 workers worked each month on the project, which was begun in 1931 and 
completed in 1935, and renamed from the original name Boulder Dam to Hoover Dam, in 
honor of the 31st US President (Weimer Purkey, 1994). 
Lake Mead (Figure 3C) with its peak surface area of 640 km
2
 represents 90  of the 
surface water in southern Nevada (Lochead, 2023). Only two percent of the flow of the 

74 
 
Colorado River as it passes through Lake Mead is contractually allocated to southern Nevada 
(Lochead, 2023). This amounts to 12 m
3
/s which satisfies about 90  of demand for the Las 
Vegas Valley (Luke et al., 2022). 
The Lake Mead reservoir provides water for 25 million people, as well as agricultural and 
natural ecosystem needs for Nevada, California, Arizona, and northern Mexico (Luke et al., 
2022). If the lake was full of water, it would reach an altitude of 374 m and a maximum 
depth of 162 m. However, the level of the reservoir has been decreasing sharply and in June 
2022 reached an altitude of 317 m above sea level, which means a decrease of 56 m from 
the highest water level, reduction in volume retained by about 70%, and the lowest level 
since 1936, when it was first filled (Luke et al., 2022). 
Flooding in August and early September 2023 made only a minor contribution to the rise 
in Lake Mead, as 90-95% of the water for the Colorado River comes from snowmelt in 
mountains far to the north. A rise of the lake level by about 6 m in the period from March to 
September 2023 is attributed mainly to releases from the upstream Glen Canyon Dam which 
forms Lake Powell (Lochead, 2023). 
 
 
Grand Canyon 
 
On August 22, 2023, an interesting adventure awaited us in the Grand Canyon South Rim 
area, where we went after the main part of Tropical Storm Hilary passed through the Las 
Vegas area. 
In the small town of Tusayan, which has about 600 inhabitants, lies at an altitude of 2,015 
m, and is about 9 km from the Grand Canyon South Rim, we experienced a road closure due 
to flooding. On the afternoon of August 22, 2023, we visited the Grand Canyon National 
Park, where it rained from time to time, and it was heavily cloudy in the east, which allowed 
us to take beautiful photos (Figure 3D). When we wanted to return to the motel in Tusayan 
around 06:00 p.m., the main road through the place was already under water. Since there 
was not much rain in the area, some people thought it was a leaking water pipe. 
In fact, 50-76 mm of rainfall fell within a few hours (2-3 h) in the Coconino Wash area 
about 10 km east of Tusayan (Sutton et al., 2023), causing flooding on the lowest part of 
Route 64 which passes through Tusayan. In Tusayan some motels and other infrastructure 
elements were also flooded, and there was no electricity. The Grand Canyon South Rim exit 
on Route 64 was closed until 9:30 p.m. on August 22, 2023, when the water receded. The 
inhabitants of this place did not recollect similar floods. 
 
 
Concluding thoughts 
 
The so-called monsoon rains in the SW part of the USA are a regular occurrence in the 
summer and early autumn, but in August 2023 the intensity of precipitation increased 
significantly, as for the first time since 1965, a tropical storm or hurricane warning was 
issued for the desert areas of Southern California. 
The effect of El Niño on the warming of the Pacific Ocean is estimated to affect the 
climate of the entire planet. Extreme weather conditions are being recorded in many places 
around the world: At the beginning of August 2023, Slovenia was hit by its worst weather 
disaster (Wikipedia, 2023b), as 200 mm of precipitation (150-200 L/m
2
) fell over 12 hours 
in some places. At the beginning of September 2023, a record amount of precipitation fell in 
a few hours in parts of Greece. In the same month, the collapse of a dam in Libya caused a 
disaster of epic proportions after heavy rains. All this demonstrates to us that so-called 

75 
 
hundred-year floods are now occurring more frequently and, very intensively, affecting 
larger areas. 
During our bilateral exchange in Nevada (as well as in the Grand Canyon, Arizona), we 
received multiple audio and text alerts on our cell phones that more rain and wind were 
approaching. 
Simultaneously with Hurricane Hilary, on August 20, 2023, a magnitude 5.1 earthquake 
with numerous aftershocks was recorded in Southern California in the vicinity of Los 
Angeles. The term "hurriquake" appeared on social networks (Wikipedia, 2023a). Although 
the two events are not related, the earthquake caused a lot of speculation about the connection 
with the extreme weather. 
The personal experience of floods and the adaptation of field work to weather conditions 
were an interesting experience that, from the point of view of study of natural phenomena 
on a dynamic planet, complemented the bilateral exchange oriented to karst geology. 
 
 
Acknowledgements 
 
The field work was made possible with the help of the bilateral project "Micro-climate 
monitoring in tourist caves, comparison of Grand Canyon Caverns with Slovenian caves" 
(ARIS BI-US/22-24-074), with the help of the infrastructure program "Co-financing of the 
implementation of the international infrastructure project EPOS" (ARIS I0-E017) and the 
research program "Karst Research" (ARIS P6-0119). 
 
 
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