How do Management Strategies Affect Flash Flooding in the Morongo Basin, California

Image Credit: Morongo Basin Conservation Association

The Morongo Basin

Abstract

The Morongo Basin, which is on the western edge of the Mojave Desert, is prone to flash flooding events. This project seeks to answer the question on how different flash flood management strategies have affected the severity of flash flooding in the Morongo Basin. Some of these strategies include building more dams and reservoirs, building channels to carry water, reducing the amount of urbanization in flood zones, preparing the community, and utilizing "green" infrastructure, the "use of vegetation for civil engineering purposes" (Hervás et al., 1970). For this study, I used a Digital Elevation Model (DEM) to perform a hydrology analysis in the study area of the Morongo Basin. This produced information on where the most dangerous areas of flash flooding would occur in heavy rains, as well as the areas of infrastructure that may be prone to damage due to the flooding. I then compared the hydrology map to a map of current "built" waterways (waterways that the city has created or modified rather than naturally produced). This study showed that each of the three major towns, Yucca Valley, Joshua Tree, and Twentynine Palms are at different levels on producing waterways to help redirect the floodwaters. Due to the desert's ecosystems and native plant species, creating a "green infrastructure" is not used, and the community preparedness, while available, could be more widespread during the peak flooding months.

Introduction

The Morongo Basin, located in eastern San Bernardino County, in Southern California, lies between Joshua Tree National Park and the US Marine Air Ground Combat Center in Twentynine Palms. It is a "fragile and diverse ecosystem of Joshua Tree forests, ancient plants, unique wildlife, mountains, and dry lake beds located in the Mojave Desert of Southern California" (Sonoran Institute). As part of the Mojave Desert, the Morongo Basin does not see a lot of rain, but is still prone to large sudden downpours. When these happen, flash flooding can have a major impact on property, infrastructure, and industry. Flash floods also "account for more than 5,000 deaths annually on a global basis" (Modrick & Georgakakos, 2015). Since the Morongo Basin includes the Northern entrances to the Joshua Tree National Park, there are a lot of visitors who come from all over. In the Morongo basin alone, there are at least 1,300 active, permitted Short Term Rentals, according to the County of San Bernardino. While these visitors most likely see more rain throughout the year than the basin does, most do not fully grasp how much water desert monsoons can drop in an area. This then brings fast moving water that turn into flash flooding. The time of the year that the basin experiences these sudden downpours is usually between July and September, or the monsoon season. "Each monsoon season, a driver in the Morongo Basin gets stuck or stranded on a road because of rapid flood waters" (Z1077). While the area does not experience flooding that reaches rooftops, flash flooding in the desert results in a large amount of fast moving water and desert soils. In 2019, an incident occurred where "The force of the water pushed the car and flipped it over about 50 feet downstream" (Z1077). This water causes a major danger to lives as well as infrastructure and desert soil erosion. The fast moving water can easily pick up the dry top layer of sand, eroding out large areas of desert floor. Despite this, the towns of Yucca Valley, Joshua Tree, and Twentynine Palms have some measures in place to reduce the effects of flash floods.

Image Credit: Gina Ferazzi - Los Angeles Times

Minh Dang observes the damage to the parking lot of his Twentynine Palms business after a monsoon swept through.

Image Credit: The Desert Trumpet

A car stranded in floodwaters in Highway 62 in Twentynine Palms

Image Credit: Z1077 News

Floodwaters brought lots of eroded soils across Highway 62, closing it down.

Photo by Jennifer Jungwirth - Joshua Tree, CA

Eroded area of desert soil from a major floodpath

Data and Methods

My study area included what is generally known as the Morongo Basin. This included the towns of Yucca Valley, Joshua Tree, and Twentynine Palms, California. The major highway connecting these three towns is Highway 62, or Twentynine Palms Highway, which connects to Interstate 10 to the West.

In order to understand the motion of floodwaters in the Morongo Basin, I used the Shuttle Radar Topography Mission (STRM) Digital Elevation Model (DEM) of the area from USGS Earth Explorer. This data is a raster of elevation at a 30 meter resolution taken from radar aboard the space shuttle Endeavor missions. This data allowed me to run a Hydrological Analysis for my study area through Esri's ArcGIS Pro Platform:

Using the DEM, I filled in any sinks caused by possible errors in the sensing of elevation, then created a flow direction raster. This raster was then used to create the flow accumulation raster seen below.

Finally, Using the flow direction and Flow Accumulation rasters, I reduced the flow Accumulation raster to only the streams that contained a flow accumulation value greater than or equal to 3,000. This helped show the more significant water runoff in red, and smaller runoff in green.

Finally, I created a Strahler Stream Order raster from the conditional flow accumulation raster to easily see where the water goes when major flooding happens.

Results

Photograph by Jennifer Jungwirth - Joshua Tree, CA

Photograph showing the major floodpath of stream order 4 near Vine Way in Joshua Tree on above map.

Conclusion

There is no underground storm drainage system throughout the Morongo Basin, so all rainwater that does not infiltrate into the ground becomes runoff. Deserts "often have few plants, poor soils and hidden river channels. This means when heavy rain hits, water can rapidly run off and pool on lower-lying land,” says Prof Hannah Louise Cloke OBE, hydrology expert at the University of Reading (Phelan). Despite this, the town of Yucca Valley has done well at building channels to direct floodwaters and have built several bridges for the major highway across the channels. They follow the direction and areas that were highlighted in my hydrology analysis well to direct the waters. This has helped some of the more urbanized areas of the town reduce the damage and dangers of large flash floods. However, there are still areas around the town that do experience flood runoff onto busy roads, like Yucca Trail and Highway 62. According to the Town of Yucca Valley's website, their Master Plan of Drainage was last updated in 1999, so unless there is a new plan, not available on their website, the town would really benefit into updating this plan. The town of Twentynine Palms has built one major waterway, which has helped but also has more work to do for some of their major roads, like Adobe Road, and north near the marine base. Joshua Tree has two main channels, but since it is an unincorporated village in a large county, some of the work that needs to be done does not happen. There are several areas farther away from the main town center (Park and Hwy 62) that experiences major floodwaters. The north end of this town experiences a floodpath with a stream order of 4. This is the major stream where lots of smaller streams lead into and extends all the way east through the basin.

Image Credit: What's Going on In the Morongo Basin

Damage Caused by flash flooding: A part of a road degrading from erosion caused by flash flooding

There is a large challenge of creating more "green infrastructure" to the Morongo Basin, in the hopes of reducing flash flooding. The desert is a very fragile ecosystem where the native plants "have wonderful mechanisms to survive heat, dry conditions, and poor soil" (Mojave Desert Plants). Bringing in lots of non-native vegetation could potentially introduce invasive species to the desert. "Native plants are a critical part of a sensitive desert ecosystem, providing food and shelter for wildlife and cleaning our air and water through soil stabilization and carbon sequestration" (MDLT). This will then cause much more damage than the floodwaters. Further, "landscape-scale restoration efforts in the Mojave Desert have low success at restoring the previous plant assemblage and shrub cover and become increasingly dominated by invasive annual plants" (Chambers et al.). To add to the damage caused to the native plant population, research shows that a "displacement of native plant communities is a key cause of a collapse in insect populations and is affecting birds as well" (Tallamy et al.). Due to these, creating a green infrastructure is neither beneficial or cost effective in the long run.

Image Credit: National Park Service

The native desert landscape in the Morongo Basin

As a new resident, I was quickly informed on understanding the effects of flash flooding, and know which roads and areas receive dangerous flooding. Groups on social media accounts like "What's Going on in the Morongo Basin" on Facebook constantly post about the damage and locations of flash flooding. To bring effort to explaining and warning about the dangers of flash flooding to the visitors or new residents, the local radio station plays reminders every monsoon season. It could be better, since there are always stories and accidents involving a driver getting stuck in flooded roads. Some residents feel that the towns are not doing enough to help reduce the effects. According to a story by the LA Times, "some expressed frustration with the city for what they described as a lack of communication to affected residents in the immediate aftermath. Others wondered whether authorities could have done more to monitor the development of private properties that they believe may have intensified the flow of water into certain areas."

Screenshot of a What's Going on in the Morongo Basin User

Overall, more effort could be taken to using the management strategies available to reduce flash flooding. Master Drainage Plans can be updated with newer analyses and procedures. More can be done to prepare the community and visitors on the dangers of flash flooding. Owners of short term rentals can keep pamphlets at their rentals that help educate the visitors on the dangers of flash flooding. Since green infrastructure would not benefit the flood prone areas more than the damage it could cause to the ecosystem, other matters need to be looked into find other strategies to help reduce the dangers and effects of flash flooding in the Morongo Basin.

References:

Chambers, Jeanne C., et al. “The Southern Nevada Agency Partnership Science and Research Synthesis: Science to support Land Management in Southern Nevada.” USDA Forest Service, 2013, pp. 25–34, https://doi.org/10.2737/rmrs-gtr-303.
Feller, Walter. “Mojave Desert Plants.” Mojave Desert Plants, mojavedesert.net/plants/. Accessed 1 July 2025.
Hervás, et al. “Nedies Project: Guidelines on Flash Flood Prevention and Mitigation.” Publications Office of the EU, Publications Office of the European Union, 1 Jan. 1970, op.europa.eu/en/publication-detail/-/publication/abbf6e10-a7e8-11e7-837e-01aa75ed71a1/language-en#.
Modrick, Theresa M., and Konstantine P. Georgakakos. “The character and causes of flash flood occurrence changes in mountainous small basins of Southern California under projected Climatic Change.” Journal of Hydrology: Regional Studies, vol. 3, Mar. 2015, pp. 312–336, https://doi.org/10.1016/j.ejrh.2015.02.003.
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“Permitted STR Properties.” San Bernardino County STR Department, str.sbcounty.gov/permitted-str-properties/. Accessed 1 July 2025.
Phelan, Joe. “The World’s Driest Deserts Are Flooding, and It Has Scientists Puzzled.” BBC Science Focus Magazine, 31 Jan. 2025, www.sciencefocus.com/planet-earth/the-worlds-driest-deserts-are-flooding-and-it-has-scientists-puzzled.
“Plant Conservation.” Mojave Desert Land Trust, www.mdlt.org/plant-conservation. Accessed 1 July 2025.
Tallamy, Douglas W., et al. “Do non‐native plants contribute to insect declines?” Ecological Entomology, vol. 46, no. 4, 17 Nov. 2020, pp. 729–742, https://doi.org/10.1111/een.12973.
“This Week in Hi Desert Podcasts – Flash Floods, Rattlesnakes, and G-L-O-R-I-A.” Z1077 FM, 27 Aug. 2022, https://z1077fm.com/this-week-in-hi-desert-podcasts-flash-floods-rattlesnakes-and-g-l-o-r-i-a/. Accessed 30 June 2025.
Wigglesworth, Alex. “Monsoon Flooding Devastates Mojave Desert Town - but Also Reveals Its Heart.” Los Angeles Times, Los Angeles Times, 7 Aug. 2024, www.latimes.com/environment/story/2024-08-07/twentynine-palms-monsoon.

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