Colorado River Delta

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Overview

The Delta before the early 1900s

Prior to the construction of dozens of large dams on the mainstem and tributaries, the Colorado River carried million tons of sediment annually to its lowest reaches[1], building a broad alluvial plain from the Salton Sea to the Gulf of California.[2] At the southern end of this plain, close to sea level and the Gulf, the Colorado River branched out into multiple distributary channels, creating over 3,000 square miles of freshwater, brackish, and saltwater wetlands: the Colorado River Delta. This shifting mosaic of verdant wetlands was sustained by the river’s inflows, which intermingled with seawater carried upstream by the tides[3].

Figure 1. Satellite imagery of the Colorado River Delta region overlain with major geographic features. The dashed blue line shows the approximate course of the Colorado River, which today flows only intermittently below Morelos Dam, mainly from irrigation return flows and also periodic environmental releases. (Imagery: Google Earth)

Thousands of Cucupá (now also known as Cocopah or Kwapa) people lived on the higher ground of the Delta, farming corn and other crops in seasonally flooded fields and fishing in the channels and the lagoons.[4] European visitors to the Delta from the 1500s onward remarked on the power of the tides and the confounding physiography of multiple channels and soft sediments that hindered upstream travel by boat and by foot.[5] It was not until the early 1900s that visitors, most famously Aldo Leopold, began to document what the Cucupá had long known about the Delta: Its dynamic landscape of wetlands supported numerous and diverse plants and animals. The Delta, Leopold wrote, held “a verdant wall of mesquite and willow…a hundred green lagoons…fleets of cormorants” and many other birds, beavers, and jaguars.[6] The Delta’s waters hosted an equally prolific aquatic fauna, including billions of endemic Colorado delta clams.[7]

The 20th-century drying of the Delta

By the time of Leopold’s exploration in 1922, the ecohydrology of the Delta was already being impacted by water use upstream. Then the closing of Hoover Dam (Lake Mead) in 1935, Morelos Dam (Mexico’s mainstem diversion) in 1950, and Glen Canyon Dam (Lake Powell) in 1963 progressively cut off virtually all of the water and sediment that formerly fed the Delta.[3] By the late 1970s, only a few Cucupá still lived in the Delta, and over 90% of the original wetlands had been replaced by agricultural fields, invasive plant species, or most conspicuously, dewatered and barren sediments. At that time, many observers wrote off the Delta as a “dead” ecosystem, even as a 40,000-acre wetland complex called the Cienega de Santa Clara emerged in the upper Delta, incidentally created by new flows of brackish agricultural drain water from Arizona.[8]

In 1980, Lake Powell finally filled, shortly followed by an unusually wet period in the mid-1980s, a wintertime flood on the lower Colorado in 1993, and then a smaller “pluvial” in the late 1990s.[9] Flows into the Delta surged, even reaching pre-Glen Canyon Dam levels in a few years, temporarily bringing life back to parts of the Delta.[9] The rapid ecological response to these higher flows and the persistence of the Cienega de Santa Clara indicated the potential for long-term restoration of the Delta if even modest inflows could be secured [10]. In 1996, environmental groups on both sides of the border began holding workshops in communities to promote restoration of the Delta.[11] In 2000, the United States and Mexico signed Minute 306, acknowledging for the first time the value of the riparian and estuarine ecology of the Colorado River Limitrophe (the river reach below Morelos Dam) and Delta to both countries.[12]

Restoration efforts in the 21st century

Since 2000, restoration efforts in the Delta have had to contend with truly unfavorable hydrology in the lower Colorado River, with persistent drought, warming temperatures, and declining reservoir levels in Powell and Mead.[1] Virtually no water reached the delta from the mainstem in the first 13 years of the new century.[9] In 2010, the two countries signed Minute 316 and committed to share with a binational coalition of non-governmental organizations (NGOs) the temporary delivery of water to the Cienega de Santa Clara, during a period when its water supply would otherwise be diminished. This was the first planned environmental water delivery to the Delta.[13] In 2012, the U.S. and Mexico signed Minute 319. Among several other provisions, Minute 319 established a 5-year pilot program to intentionally deliver water for environmental purposes to the Delta, most of it in a single “pulse flow.”[14] The pilot program was to be collaboratively implemented by the U.S. and Mexican federal governments and a binational coalition of NGOs.

After much planning, and under extensive scientific monitoring, 105,000 acre-feet of water was released from Morelos Dam and two other structures downstream from March to May 2014.[15] The pulse flow wetted the dry riverbeds of the Delta for more than 80 miles, with a small amount of water eventually reaching the estuary at the northern end of the Gulf of California.[16] While a tiny fraction of the volume and duration of the pre-dam spring floods,[17] the 2014 pulse flow still promoted the growth and establishment of trees and shrubs, led to increased bird abundance and diversity, recharged the aquifer, and brought cultural and societal benefits.[16] The pulse flow was followed up with intensive planting and restoration along the river corridors. In 2017, Minute 323 was signed, facilitating additional seasonal releases of water through 2026 to benefit the environment; releases of 20,000 to 35,000 acre-feet over 5-6 months occurred in 2021, 2022, and 2024, and are planned for 2025 and 2026.[18] Minute 323 also set the stage for a binational project to improve a wastewater treatment plant in Mexicali, which when completed will send 11,000 acre-feet of treated water to the Delta each year.[6]

The Delta today

Despite the much-degraded condition of the Delta’s ecosystems, covering less than 10% of their original extent, they remain important habitat to diverse populations of birds and other animals, including several federally listed endangered or threatened species (in U.S. and/or Mexico): Yuma Ridgway’s Rail (Rallus obsoletus yumanensis), Southwestern Willow Flycatcher (Empidonax traillii extimus), Yellow-billed Cuckoo (Coccyzus americanus), and desert pupfish (Cyprinodon macularius). The Delta is also an important oasis for migrating birds on the Pacific Flyway; over 250 species use the Cienega de Santa Clara alone, and over 350 species have been documented across the Delta.[19] The Delta also remains important to the cultural heritage of the Cucupá and other Mexican people who live in towns and villages along the lowermost river and around the Delta.[11] Many of these local communities have joined the restoration efforts.

Future efforts to restore and maintain riparian and wetland habitat in the Delta could face even greater challenges from hydroclimatic conditions. A continuation of the Upper Basin hydrology seen from 2000-2024 would likely preclude any releases of surplus water from Lake Mead and Morelos Dam into the Delta, as occurred in the 1980s and 1990s, and additional warming will further stress the vegetation in the Delta trying to hang on between environmental releases. Minute 323 expires in 2026, and any subsequent agreements will need to accommodate the uncertain contours of the Post-2026 Colorado River operations.

Data and tools

USGS Colorado River Delta Minute 319 monitoring data

This dataset includes compilation of vegetation indices, phenology assessment metrics, estimates of evapotranspiration, and change maps for seven reaches of the delta's 150-km long region, for almost a 20-year period from 2000-2019.


Additional resources

Environmental Work Group reports for Minute 319 and Minute 323

These reports document the binational effort to monitor and study hydrologic, vegetation, and fauna responses to the releases of periodic environmental flows, starting with the 2014 Pulse Flow.

Minute 319 - Final Report (2014-2017)

Minute 323 - 2018 Interim Report

Minute 323 - 2019 Implementation Report

Hydrologic monitoring in the Colorado River Delta - Sonoran Institute

A very detailed StoryMap documenting an environmental flow through the Delta in 2021 and the restoration and monitoring activities before and during the flow.


Notes

  1. ↑ Rebecca J. Dorsey, Greg Lazear; A post–6 Ma sediment budget for the Colorado River. (2013). Geosphere 9 (4): 781–791. doi: https://doi.org/10.1130/GES00784.1
  2. ↑ Ross, J. E. (2020). Formation of California’s Salton Sea in 1905–07 was not “accidental.” The 2020 Desert Symposium Field Guide and Proceedings, 217–230. http://www.desertsymposium.org/DS%202020%20Changing%20Facies%20for%20web%204.pdf
  3. ↑ 3.0 3.1 Briggs, M., Congdon, C., Glenn, E., Luecke, D. F., Pitt, J., & Valdes-Casillas, C. (1999). Delta Once More: Restoring Riparian and Wetland Habitat in the Colorado River Delta. https://www.edf.org/sites/default/files/425_delta.pdf
  4. ↑ Kelly, W. H. (1977). Cocopa Ethnography. University of Arizona Press, 160 pp. https://open.uapress.arizona.edu/system/actioncallout/c/6/c/c6c59819-3b16-44a0-b565-29a1c6acdb2c/attachment/77e487bf5bd9918b6df8ef00d403d94c.pdf
  5. ↑ Sykes, G. (1937). The Colorado Delta. Carnegie Institution of Washington and American Geographical Society of New York. https://babel.hathitrust.org/cgi/pt?id=uc1.31822010629376&seq=7
  6. ↑ 6.0 6.1 Isakowitz, L. (2019). Restoring the Colorado River Delta. The Nature Conservancy website. https://www.nature.org/en-us/about-us/where-we-work/priority-landscapes/colorado-river/restoring-the-delta/
  7. ↑ Smith, J. A., Auerbach, D. A., Flessa, K. W., Flecker, A. S. and G. P. Dietl. (2016). Fossil clam shells reveal unintended carbon cycling consequences of Colorado River management. Royal Society Open Science 3160170. http://doi.org/10.1098/rsos.160170
  8. ↑ Glenn, E. P., Felger, R. S., Burquez, A., and D. S. Turner. (1992). Cienega de Santa Clara: Endangered Wetland in the Colorado River Delta, Sonora, Mexico. Natural Resources Journal 32, 817. https://digitalrepository.unm.edu/nrj/vol32/iss4/
  9. ↑ 9.0 9.1 9.2 Mueller, E. R., Schmidt, J. C., Topping, D. J., Shafroth, P. B., Rodriguez-Burgueno, J. E., Ramirez-Hernandez, J., & Grams, P. E. (2017). Geomorphic change and sediment transport during a small artificial flood in a transformed post-dam delta: The Colorado River delta, United States and Mexico. Ecological Engineering, 106, 757-775. https://doi.org/10.1016/j.ecoleng.2016.08.009.
  10. ↑ Pitt, J., Luecke, D. F., Cohen, M. J., and E. P. Glenn. (2000). Two Nations, One River: Managing Ecosystem Conservation in the Colorado River Delta. Natural Resources Journal 40 819. https://digitalrepository.unm.edu/nrj/vol40/iss4/4
  11. ↑ 11.0 11.1 Sonoran Institute. (2007). Colorado River Delta Restoration: 10 Years Past and Future. 4 pp. https://sonoraninstitute.org/files/pdf/colorado-river-delta-restoration-10-years-past-and-future-03152007.pdf
  12. ↑ International Boundary and Water Commission (IBWC) United States and Mexico. (2000). Minute No. 306: Conceptual Framework for United States-Mexico Studies for Future Recommendations Concerning the Riparian and Estuarine Ecology of the Limitrophe Section of the Colorado River and its Associated Delta. December 2000, 5 pp. https://www.ibwc.gov/wp-content/uploads/2023/05/Min306.pdf
  13. ↑ International Boundary and Water Commission (IBWC) United States and Mexico. (2010). Minute No. 316: Utilization of the Wellton-Mohawk Bypass Drain and Necessary Infrastructure in the United States for the Conveyance of Water by Mexico and Non-Governmental Organizations of Both Countries to the Santa Clara Wetland During the Yuma Desalting Plant Pilot Run. April 2010, 11 pp. https://www.ibwc.gov/wp-content/uploads/2023/03/Minute_316_w_JR.pdf
  14. ↑ International Boundary and Water Commission (IBWC) United States and Mexico. (2012). Minute No. 319: Interim International Cooperative Measures in the Colorado River Basin through 2017 and Extension of Minute 318 Cooperative Measures to Address the Continued Effects of the April 2010 Earthquake in the Mexicali Valley, Baja California. November 2012, 19 pp. https://www.ibwc.gov/wp-content/uploads/2012/11/Minute_319.pdf
  15. ↑ Pitt, J., and E. Kendy. (2017). Shaping the 2014 Colorado River Delta pulse flow: Rapid environmental flow design for ecological outcomes and scientific learning. Ecological Engineering. 106. https://doi.org/10.1016/j.ecoleng.2016.12.002
  16. ↑ 16.0 16.1 Flessa, K., Kendy, E., and K. Schlatter. (2016). Minute 319 - Colorado River Limitrophe and Delta Environmental Flows Monitoring Interim Report. For the International Boundary and Water Commission, May 2016, 78 pp. https://www.ibwc.gov/wp-content/uploads/2023/06/2016_EFM_InterimReport_Min319.pdf
  17. ↑ Kendy, E., Flessa, K. W., Schlatter, K. J., de la Parra, C. A., Huerta, O. M. H., Carrillo-Guerrero, Y. K., and Guillen, E. (2017). Leveraging environmental flows to reform water management policy: Lessons learned from the 2014 Colorado River Delta pulse flow. Ecological Engineering, 106, 683-694. https://pdf.sciencedirectassets.com/271742/1-s2.0-S0925857417X00104/1-s2.0-S0925857417300691/dx.doi.org/10.1016/j.ecoleng.2017.02.012
  18. ↑ Bairstow, L. (2024). Strategic water deliveries return to the Colorado River Delta. Blog post on "Raise the River," dated May 21, 2024. https://raisetheriver.org/2024-21-may-strategic-water-deliveries-in-the-delta/
  19. ↑ Hinojosa-Huerta, O., Soto-Montoya, E., Gómez-Sapiens, M., Calvo-Fonseca, A., Guzmán-Olachea, R., Butrón-Méndez, J., et al. (2013). The birds of the Ciénega de Santa Clara, a wetland of international importance within the Colorado River Delta. Ecological Engineering, 59, 61-73. https://www.sciencedirect.com/science/article/abs/pii/S0925857413001092


Page credits:

  • Author: Jeff Lukas, Wiki Team
  • Reviewer: Jennifer Pitt, Colorado River Program Director, Audubon

Last update: July 2025

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