The Santa Cruz River begins in Southern Arizona, flows into Mexico, then turns and flows north back into Arizona through Tucson, and joins the Gila River south of Phoenix, a tributary of the Colorado River. The river is about 200 miles long. The riverbed is often dry, but this is partly due to both natural and human actions in the nineteenth century, including overgrazing, excessive pumping of groundwater for agricultural irrigation and industry, and the construction of dams and ditches. Since 2019, releasing treated wastewater into the river has resulted in higher flow rates along parts of the river.
According to Wikipedia:
“In June 2019, the city of Tucson began releasing
treated wastewater daily into the Santa Cruz River bed near West Silverlake
Road as part of the Santa Cruz River Heritage Project. This has resulted in
renewed perennial flow in an approximately 1-mile (1.6 km) stretch near
downtown Tucson. Further upstream, perennial flows returned to a half-mile
stretch of the river through the San Xavier Indian Reservation of the Tohono Oʼodham
Nation in 2019 as a result of reduced ground water pumping due to greater
availability of water from the Central Arizona Project. This has led to the
revival of vegetated riparian zones along the river, including areas of
cottonwood shoots and seep-willow. Further downstream at the Roger Road Waste
Water Treatment Plant has been releasing treated wastewater, which extends the
perennial flow of the river for approximately 46 miles (74 km).”
Indigenous people created an
extensive system of irrigation canals, supporting an agricultural society that
lasted thousands of years. From the 1600s and through the 19th century, first
Spanish explorers and then Anglo immigrants all depended on the river. The area
is very dry and part of the Sonoran Desert. As a result of decades of
groundwater pumping, often for irrigation, the water table, once at 25 ft below
the surface, is now closer to 300 ft below the surface. That led to the local
rivers drying up and not flowing year-round like they used to do.
The restoration project,
known as the Santa Cruz River Heritage Project, began releasing treated
wastewater into the river in 2019. The treated water does not meet drinking
water standards but does meet effluent standards for treated water. One hope of
the restoration is to get the river to flow year-round as it once did in the
1900s.
A Section of the River Showing Increased Flow and Clearer Water Downstream from the Introduced Cleaner Effluent
According to Tucson Water:
“In the early 2000s, Tucson Water began storing and
recovering potable water from the Colorado River via the Central Arizona
Project canal — further alleviating aquifer overdraft from the well fields that
served most customers.”
Restored water flow on the
river has led to riparian habitat revival with the return of native plant
species and endangered wildlife. In fact, the return of native species like
dragonflies, damselflies, mayflies, toads, snakes, birds, and the federally
endangered Gila topminnow has been faster and more intense than predicted. The
return of cottonwood, mesquite, and willow woodlands along the river’s banks
has been a welcome change.
“Tucson Water adds chloramine as a biological control
for the reclaimed water system. For the Heritage Project, Tucson Water began
releasing Class A-treated recycled water into the Santa Cruz River from a newly
constructed valve station built onto existing reclaimed water infrastructure.
Sodium bisulfite is added at the valve station to remove any residual traces of
chlorine.”
“A small treatment facility near the bank of the river
dechlorinates flows and is metered to provide real-time monitoring of water
quality.”
Before the heritage section
of the river was restored near Tucson, treated wastewater was first added
upstream. In 2013, the Pima County Regional
Wastewater Reclamation Department constructed the Agua Nueva Water Reclamation
Facility to replace former treatment facilities. The Agua Nueva facility
produces effluent that is safe for all uses, excluding body-contact recreation
and consumption.
Another goal of the project
is to recharge local groundwater aquifers, hopefully reversing the considerable
groundwater depletion that has dropped the water table considerably. Below are
some goals and highlights of the project from the U.S. Fish and Wildlife
Service (USFWS).
USFWS also notes that challenges remain due to non-native species:
“Nonnative vegetation including Tamarix spp., bermuda
grass, buffelgrass (Cenchrus ciliaris), and Johnson grass (Sorghum halepense),
and nonnative aquatic species including western mosquitofish (Gambusia affinis)
and American bullfrog (Lithobates catesbeinas) pose ongoing management
challenges in the Heritage Reach.”
The river’s restoration has
been well-received by the local public. A recent paper in the journal
Restoration Ecology explored the project, noting that using treated wastewater
to restore rivers was a new idea and needed further analysis. The authors
verified the fast and thorough return of native species. However, they did
caution that more precise flow increases to key areas would be needed to
restore some sensitive species. The paper’s abstract is below.
“Simply adding effluent to dewatered urban rivers has
the potential to restore diverse aquatic fauna, but targeted reintroductions
may be needed for sensitive or dispersal-limited taxa.”
Dredging on the river for
flood control near Tucson, where flows can increase quickly due to rain, gave
the researchers a chance to see the effects of a temporary flow stop, and
another quick return of species was observed when effluent flow was returned.
The region, though very dry, does have a monsoon season where floods can and do
occur.
A Section of the Santa Cruz River After Heavy Monsoon Rains
Pima County spent $600
million to upgrade two wastewater treatment plants, which output cleaner water
than the previous treatment plants. Some species were introduced or
reintroduced to the area. The researchers noted:
“Effluent is an important and underappreciated tool for
creating new habitat for many riverine species in arid and semiarid regions.”
It is widely acknowledged
that if effluent from the treatment plants before the upgrades were released,
there would not be restoration, which is dependent on the better water
treatment provided by the upgrades. Reduced levels of nitrogen and other
organic compounds in the effluent flowing into the Santa Cruz River after
cleaner effluent was introduced was key to the return of species. A mass
die-off of cottonwood trees that occurred along the river’s banks in 2005 was
seen as a wake-up call. Research and monitoring on the river intensified in
2008. Reduced nitrogen levels also increase infiltration into groundwater and
could account for reduced flows in some parts of the river.
The Sonoran Institute has
been a key partner in the project with a huge database on river parameters.
“Together with Pima County, the Sonoran Institute since
2013 has been compiling Living River reports on the Lower Santa Cruz River, a
23-mile stretch northwest of Tucson. Modeled on those for the Upper Santa Cruz,
these reports began with baseline measurements of conditions prior to the
treatment facility upgrades, and have continued with annual assessments,
tracking water quality, native fish, wildlife, and the overall ability of the
public to enjoy the river.”
Before the upgraded effluent
was introduced, the river was often plagued by rotten egg smells in Tucson,
which no one enjoys, including plant and animal species.
“Thanks to the effluent that flows out of the nearby
Nogales International Wastewater Treatment Plant, the Santa Cruz River in this
stretch has surface-flowing water for most of the year, supporting an abundant
diversity of plants and animals, including a cottonwood-willow forest, one of
the rarest forest types in North America. It provides habitat for mountain
lions, coyote, bobcat, deer, coatimundi, and javelina. It is also part of what
Audubon Arizona has named an “Important Bird Area” for the density and
diversity of birds it supports, including nesting Yellow-billed cuckoos, a
species of conservation concern. All of this life has made the park and
connecting Juan Bautista de Anza Trail, the two publicly accessible parts of
the river in this area, increasingly popular with hikers, equestrians, and
birders.”
The return of the endangered
Gila topminnow is an important biological indicator that the river is cleaner.
Mexico has domestic rights to
the river in its upstream reaches, which can affect flow into the U.S. They
have also been upgrading wastewater treatment plants to get better quality
effluent into the river. Collaboration is ongoing, but there is still a threat
to the river as other sources of contaminants, like grazing cattle and nutrient
runoff, still threaten the river’s water quality. Stephanie Sklar, CEO of the
Sonoran Institute, noted:
“We’ve spent over 25 years working on the Santa Cruz
River on both sides of the border, so the river is part of our DNA.”
Although the success will
need to be further monitored and ensured, the restoration of the Santa Cruz
River via cleaner treated wastewater could be used as a model for other
threatened rivers with low flow in desert and arid areas.
References:
Turning
Reclaimed Water Into Wildlife: How Tucson Saved The Santa Cruz River (Twice). Water
Online. December 6, 2024. Turning
Wastewater Into Wildlife How Tucson Saved The Santa Cruz River (Twice)
Scientists
bring Arizona river, biodiversity back from the brink — by adding sewage water.
Kamrin Baker. Good Good Good. September 3, 2024. Arizona
river sees biodiversity boom after scientists add wastewater
Ecological
Response to Restored Flows in the Urban Santa Cruz River: Case Study by the
Conservation and Adaptation Resources Toolbox: Status: Completed. U.S. Fish and
Wildlife Service. Response to
Flows in the Santa Cruz River | U.S. Fish & Wildlife Service
Tucson
Water revitalizes Santa Cruz River. World Water, March/April 2020. Tucson. scrhp_article_world_water.pdf
Santa
Cruz River: Paradise Lost, Paradise Reborn, Will it be Lost Again? The Sonoran
Institute. 2025. Santa Cruz
River: Paradise Lost, Paradise Reborn
Santa
Cruz River (Arizona). Wikipedia. Santa Cruz
River (Arizona) - Wikipedia
Macroinvertebrate
community development and resilience to channel dredging following flow
restoration using effluent in an urban desert river. Michael Bogan, Hamdhani
Hamdhani, Drew Eppehimer, Kelsey Hollien, and Brian Gill. Restoration Ecology. Volume33,
Issue2. Special Issue: RE‐AL Thematic Series. February 2025. Macroinvertebrate
community development and resilience to channel dredging following flow
restoration using effluent in an urban desert river - Bogan - 2025 -
Restoration Ecology - Wiley Online Library
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