Rehabilitating the Colorado River Delta

Published: July 18, 2025 • By Cody Clemens

Aerial view of river channels and tributaries branching through a dry, barren landscape, leading towards a distant body of water.

National Geographic. https://www.nationalgeographic.com/science/article/141216-colorado-river...

Executive Summary

Although once a thriving habitat, the Colorado River Delta (Delta) has been reduced to a fraction of its original size. Nearly nonexistent water flows from upstream have caused much of the Delta to dry, removing habitat depended on by multiple species endemic to the area. Environmental flows offer a potential strategy for the rehabilitation of the Delta by returning water to areas that need it most. Securing the amount of water necessary to implement a comprehensive solution will be the biggest challenge moving forward.

Introduction

At its peak, the Colorado River Delta was home to lush ecosystems, including riparian, marsh, and estuarian habitats (Glenn et al., 2013). Connecting from its headwaters in the Rocky Mountains of Northern Colorado, the Colorado River runs through Utah, Arizona, Nevada, California, and finally Mexico, before flowing into the Gulf of California. Since the early 1900s, engineering projects to dam and divert the Colorado River have severely altered the natural flow of water to the Delta (Flessa et al., 2013). Historical overallocation of the Colorado River for agriculture and municipal use (Kuhn & Fleck, 2019) paired with decreased flow due to rising temperature associated with climate change (Stanley, 2019) means water no longer naturally discharges into the Delta. Any water that does eventually flow through the Delta is either a result of local precipitation or surplus entering from irrigation return channels (Flessa et al., 2013).

Aquatic ecosystems in the Delta rely on both freshwater and brackish water flows to function properly (Glenn et al., 2013). Since the 1980s, any freshwater flows reaching the Delta originated from the unintentional overflow of freshwater reservoirs, or from large, infrequent flooding events. This changed in 2012 when the International Boundary and Water Commission (IBWC) agreed to supply an environmental flow to the Delta in the spring of 2014, commonly referred to as Minute 319. Minute 319 is an experiment to observe how the Delta’s various ecosystems respond to a single pulse flow of roughly 105,000 acre-feet. (Flessa et al., 2013) Thorough investigations into the ecological benefits of Minute 319 have been performed and offer insights into effective rehabilitation strategies for the future.

A whole, silver-grey fish with a streamlined body, a pointed head, and a forked tail, displayed on a white background.Totoaba Fish

Totoaba Macdonaldi

The totoaba is a large schooling fish endemic to the Gulf of California that is listed as vulnerable on the IUCN Red List (IUCN, 2021). During winter, adults travel north to spawn in the Colorado River Delta, where their eggs are carried further up the Delta by tides (Giffard-Mena et al., 2020). Totoaba larvae evolved in an estuarian habitat, depending on freshwater flow from upstream to lower the water’s salinity. The survival rate of larvae is significantly diminished when they are placed in areas of increased salinity (Giffard-Mena et al., 2020). For this reason, maintaining freshwater flow into the Delta is critical for the persistence of the totoaba offspring, and thus, the overall survival of the species.

Environmental Flow

The “quantities, quality, and patterns” of water flows necessary to maintain both aquatic ecosystems and the services they provide (Acreman et al., 2014).

Colorado Delta ClamConical seashell with a worn, textured surface in shades of beige and cream, displayed on a white background. The shell's smooth tip contrasts its layered base.

Mulinia coloradoensis

The Colorado Delta Clam is an infaunal, suspension-feeding bivalve mollusk that is endemic to the Gulf of California. Although not included on the IUCN Red List, changes in the flow of the Colorado River have reduced the species’ population size by roughly 90% since the mid-1900s. Like the totoaba fish larvae, the Colorado Delta clam cannot survive in highly saline environments and is dependent on freshwater flowing from the Delta. The remainder of the species can be found in intertidal regions at the mouth of the Delta, an area where just enough freshwater is introduced via agriculture return flows to maintain a suitable habitat for the clams. (Cintra-Buenrostro et al., 2005)

Map depicting Colorado River near the US-Mexico border, showing All-American Canal, Morelos Dam, Gila River, Sonoran Desert, and Gulf of California.

The extent of the Colorado River Delta, spanning from the Morelos Dam on the US-Mexico border and down into the Gulf of California.

Learning From Experience

The only way to rehabilitate the Colorado River Delta is to ensure adequate flows of water are provided regularly. Maintaining the base flow of the Delta supplies existing vegetation with water and supports microorganism communities living within saturated soils (Zamora et al., 2005). It also creates a more suitable environment for estuarian species that cannot handle high levels of salinity, such as the totoaba fish and the Colorado Delta clam. One issue observed during the Minute 319 pulse flow was that over 90% of the water infiltrated into the ground before reaching the halfway point between the Morelos Dam and the Gulf of California (IBWC, 2018). This was largley due to water loss from the primary channel into off-channel depressions, effectively removing the water from the main flow. To limit these losses, flow volumes Water discharge volume in the Colorado River Delta as the Minute 319 pulse flow travelled downstream from the Morelos Dam. should be reduced to match the channel capacity, which reduces spillover and increases flow duration and extent.

Along with a steady base flow, periodic pulse flows also benefit aquatic ecosystem health by washing away salts and other pollutants that have accumulated in the river’s floodplain (Zamora et al., 2005). Larger flow volumes released at shorter intervals are required to inundate areas outside the main river channel. As environmental factors are constantly changing, monitoring soil salinity and ecological responses is recommended for determining the timing, volume, and duration of a pulse flow that will maximize its ecological effectiveness. (IBWC, 2018)

One main goal for pulse flow events should be inducing a periodic erosion of the channel, and in turn, a transfer of sediment into the Delta that is no longer carried out by natural river flows. Sufficient sedimentation will change the morphology of the river and create new habitats for aquatic plants and macroinvertebrates, as well as spawning beds for certain fish species (Hauer et al., 2018). Furthermore, sedimentation transfers minerals and nutrients throughout the Delta, supporting the growth of aquatic and riparian vegetation, and ultimately facilitating increases in biodiversity as more species return to the area.

Flow chart illustrating water discharge from Morelos Dam, showing infiltration, surface discharge, and evaporation in million cubic meters over seven reaches.

Water discharge volume in the Colorado River Delta as the Minute 319 pulse flow travelled downstream from the Morelos Dam.

The Minute 319 pulse flow had a magnitude of about 4,200 ft3/s, which was not high enough to significantly erode the channel and transport sediment throughout the Delta (IBWC, 2018). Although this is unsurprising considering historical floods in the Delta had peak magnitudes up to 81,000 ft3/s, it highlights the importance of planning pulse flows with a large enough volume to ensure the intended outcomes will take place.

Water Acquisition

The main difficulty in rehabilitating the Colorado River Delta is obtaining sufficient water rights from an already overallocated system that is being further taxed by climate change. Due to an extended hot drought in the southwestern United States, the mean flow of the Colorado River has significantly decreased in the last two decades (Udall & Overpeck, 2017). Current research suggests the ongoing drought is more intense than any other experienced in the southwest for the last 1,000 years, and that the future will be significantly drier than the present (Wahl et al., 2022). Although these compounding factors have made obtaining water in the Colorado River a challenge, most states, excluding Nevada and New Mexico, have passed laws allowing the transfer of water rights specifically for environmental flows (Szeeptycki et al., 2015). These laws make it possible for water-rights holders to support conservation projects by either donating a portion of their yearly water allowance or transfering all of their water rights in perpetuity. While useful, donations are not the only avenue to obtaining water.

Minute 319 was made possible because the United States and Mexican federal governments collaborated and funded water conservation projects. Water-governing agencies implemented adjustments to water budgets and created enough surplus to meet the flow requirements of Minute 319. Operating through these water-governing agencies is beneficial for multiple reasons: First, water allocation should not be entirely dependent on the generosity of water-rights holders. It is crucial to have reliable methods of obtaining water rights to remove any uncertainty influencing a rehabilitation project’s success. Second, the agencies and employees that oversee these municipal water supplies usually have a breadth of experience in managing water rights. They already possess the knowledge and experience required to find water where none seems to exist. Third, these agencies tend to operate within large frameworks that are based around interdisciplinary, interagency, and intergovernmental cooperation, where every agency is focused on achieving a mutually desired outcome.

Conclusion

The Colorado River Delta is a severely damaged aquatic habitat capable of hosting a multitude of species and is in dire need of rehabilitation. Comprehensive rehabilitation strategies have been created based on previous environmental flows into the Delta. Depending on the desired outcome, these strategies will differ. Providing a constant base flow of water to the system allows ecosystems to reestablish themselves and continue to thrive into the future. In addition, introducing high volume pulse flows removes salts and other pollutants from the Delta while simultaneously supplying it with water and sediments. Unfortunately, the flow of the Colorado River has been decreasing for the last two decades, reducing overall water availability. Rehabilitating strategies are rendered ineffective if water-governing agencies cannot obtain sufficient water rights. Building strong relationships between governmental and non-governmental agencies on all levels is likely the best approach to securing a large portion of these water rights. Although the rehabilitation of the Colorado River Delta will be challenging, it’s an achievable goal towards which considerable progress has already been made.

 

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