USA: Arizona: North Simpson Habitat Restoration Project


The North Simpson Habitat Restoration Project is a project of Tucson Audubon Society on lands owned by the City of Tucson, Arizona. The project involves the planting of numerous native tree, shrub, grass, and forb species along a riparian area that has come into existence since the 1970s as a consequence of municipal effluent discharge. The project’s design is to restore habitat that was destroyed through groundwater pumping and urban development in the Tucson basin along a stretch of river that was formerly riparian. The site itself was not thought to be riparian historically, but the intention of project managers was to restore the structural diversity of the habitat to benefit wildlife in the area. Using volunteer labor ranging from dedicated activists to high school students, Tucson Audubon has conducted classes in restoration at the site, offered birding tours, and is in the process of monitoring the site closely to provide a clear picture of the success of their work.

Quick Facts

Project Location:
Simpson Farm, Tuscan, Arizona, 38.3573854, -98.80673580000001

Geographic Region:
North America

Country or Territory:
United States of America

Desert/Arid Land, Freshwater

Hot and Dry Desert

Area being restored:
1700 acres

Project Lead:
Tucson Audubon Society

Organization Type:
NGO / Nonprofit Organization


Project Stage:

Start Date:

End Date:

Primary Causes of Degradation

Agriculture & Livestock, Fragmentation, Other

Degradation Description

By the mid-twentieth century, the Simpson Farm site had little or no native vegetation remaining on it. Due to a combination of groundwater pumping, agricultural disturbances, increased regional development and other agricultural development pressures, the site underwent total vegetative change from native vegetation to one dominated by annual weeds and few trees.

Reference Ecosystem Description

The Santa Cruz River was once a perennial stream through the Tucson Basin. After decades of groundwater overdraft, the river eventually dried up, along with long stretches of riparian vegetation. The location of the Simpson Habitat project is at the very margin of what is historically known to have been riparian. It is thought that the reach of the Santa Cruz river that is part of the project was most likely intermittent, running during flood events only. Immediately upstream, the Santa Cruz River historically had bands of vegetation through what is now Tucson, with cottonwood-willow riparian forest immediately along the channel, and mesquite bosque in the adjacent floodplain. Similar habitat in Southern Arizona today is found to have the highest density of breeding bird populations.

Project Goals

Throughout the twentieth century, the Santa Cruz River had been a lessening stream because of groundwater pumping and subsequent arroyo cutting episodes that altered the hydrology of the river itself. The consequence of these two factors was the deterioration and disappearance of large amounts of riparian forest and adjacent mesquite bosque. The Simpson Habitat Restoration Project aimed to improve ecological connectivity to other ecologically rich ecological corridors such as the Brawley/Los Robles Wash, which passes through the Buenos Aires Wildlife Refuge in the Altar Valley and within a mile of the Simpson Site. Central to the mission of the Tucson Audubon Society was the goal to restore vital bird habitat to the Santa Cruz valley. The habitat enhancement strategy for the river stretch focuses on widening and diversifying the vegetation growing along the channel, to make more and richer wildlife habitat.


The project does not have a monitoring plan.


Beginning in the 1970s the City of Tucson began discharging treated effluent into the Santa Cruz River. In the 1970s and 1980s the City of Tucson purchased 23,000 acres of adjacent farmland for groundwater rights and subsequently retired the farmland. Tucson Audubon Society became involved in the project in 2000, signing a 99-year right-of-entry lease with the city, with the agreement to restore habitat and monitor wildlife on the initial 1,700 acres.

Description of Project Activities:
Tucson Audubon Society developed a site assessment and work plan, before they began spreading seed and planting trees and shrubs. Once the initial work plan was established, Tucson Audubon utilized a large pool of volunteers though restoration work days at the site to build irrigation systems, water harvesting swales, and to plant trees, shrubs, and grasses. Employing a wide pallet of species, work at the Simpson site has planted a wide variety of trees including: mesquite, palo verde, elderberries, desert willow, walnut, cottonwood, and willow. Habitat restoration at the Santa Cruz River Habitat Project also involved a complete plant pallet (shrubs, grasses, cacti, annuals, etc., in addition to trees) to fully restore the known historical diversity among the riparian and floodplain communities. In addition, students from Sunnyside, Tucson and Desert View High Schools and from other schools throughout Pima County were involved. Seed is pelletized by mixing it with native clay and water, and then rolling it into balls or pushing it through a mesh that breaks it up into small pieces. In this innovative technique, the clay dries around the seed and provides a protective coat that reduces predation by birds, insects, and rodents. In the next heavy rainfall the clay dissolves and exposes the seed precisely when germination conditions are most favorable. Plant species were chosen for the site based on their historical presence in the area, successful use in other restoration efforts, recommendations from experts, suitability to site soil, their benefit to wildlife, and other criteria. The mix of plant species was also important and all new plants were arranged in groups and tucked in around existing plants to create "guilds" (an association of plants that benefit each other and enrich the entire habitat). Through the use of 'restoration workshops', the staff of Tucson Audubon was able to gain hundreds of hours of volunteer labor for the project while also providing an educational outreach component to their work. Plants were irrigated so that they can be watered for several years until they can survive on their own. Since making the system self-sustaining was another important goal, plants were placed in deep earthen basins with stout downhill berms so that rainwater will be concentrated around them. The use of deep organic mulch reduces evaporation from the soil, and gradually breaks down to add nutrients. The main strategy has been to use trees and shrubs that will do well on the river berms away from direct influence of the stream. These species will broaden the swath of vegetation along the river corridor but will be able to survive on rainwater alone. These include mesquite (Prosopis velutina), blue palo verde (Parkinsonia florida), desert willow (Chilopsis linearis), graythorn (Ziziphus obtusifolia), alkali sacaton (Sporobolus airoides), and many others. The abandoned farm fields surrounding the river have very low vegetation diversity and density. Here we plant and sow seeds of a similar array of native, drylands species. The basin holding the plants also fills with water during heavy rains, focusing runoff from surrounding areas around the roots of the plant. The project also does large-scale imprinting on old farm fields. This involves pulling a cylinder implement with protrusions over the fields with a tractor, leaving a pattern of shallow indentations in the ground. Then another tractor spreads seed on the imprinted land, which tends to settle into the indentations along with finer sediments and blown organic materials. The indentations also help focus rainwater on the seeds which then have a greater chance of surviving and growing.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
To date, the work at the Simpson site has seen thousands of plants established, miles of irrigation line installed, and hundreds of acres have been seeded. Habitat has improved at North Simpson due to increased plant diversity, density and structural diversity, all enhanced by restoration efforts. Improvement has also resulted from the absence of cattle, which were fenced out in 2001. Bird surveys have shown increases in the diversity of bird species at the site, including the relocation of 24 Burrowing Owls to the Simpson site.

Factors limiting recovery of the ecosystem:
Because the Simpson site was not an historically perennial stream, recovery is a tricky term in its usage here. Because of the growth of urban Tucson, the Santa Cruz river through the city has little chance of being restored, so the habitat enhancement efforts downstream represent a replacement of what has been permanently lost as it flows through the city.

Socio-Economic & Community Outcomes Achieved

Economic vitality and local livelihoods:
The decline of the riparian vegetation along the Santa Cruz river was a phenomenon not limited to Tucson alone. Along long distances of its reach, the Santa Cruz has undergone significant vegetative change, with significant sections being completely developed in the latter half of the twentieth century. The resurgence of large sections of riparian forest is extremely valuable for bird populations in an important region for bird migration and nesting. The loss of bird diversity has coincided with 80-90 percent riparian habitat across the state of Arizona. The restoration of riparian habitat is critical for bird species throughout the arid southwest. Tucson Audubon's efforts to involve the public in restoration efforts has broadened the understanding and appeal of restoration throughout Tucson. The effort to involve a large slice of the community has increased the visibility of the project, making it valuable for educating the public about the need for further restoration in the Santa Cruz watershed.

Key Lessons Learned

One critical element of the Simpson site has been its grant-funding mandated monitoring of birds. The objective of the bird monitoring was to document changes in bird numbers and diversity. Spring and summer surveys along the 1.4-mile survey area consisted of eight point counts, an effective monitoring approach during nesting season when most birds are defending territories. On fall and winter surveys, the counts are done in a series of transects since birds may forage more broadly on the landscape in these seasons. While results based on the first four years are not statistically significant, they do indicate some trends. Since 2001, summer bird diversity increased from 37 to 51 species. The increase may not yet be related to planting and seeding efforts, as most new plants were still relatively small. Some of the increase may have had to do with the natural growth of the willow and cottonwood canopy in the absence of cattle, which were fenced out by the end of 2001. Winter flooding scoured sandy benches along the river and hastened the germination of still further new cottonwoods and willows. Yellow-breasted Chat, Bell’s Vireo, and Song Sparrow have shown notable positive numerical trends during the study period. Bullock’s Oriole and Yellow Warbler, both canopy associated species, were more variable but indicate a possible positive trend. Brown-crested Flycatcher and Hooded Oriole were first recorded in 2004. Bird diversity in the winter has also increased. Data showed an increase from 32 to 45 species detected during winter surveys. This may have occurred for reasons similar to the summer increases. Spring and fall results are more variable, showing no clear increase or decrease in species diversity. This may be due to the presence of migrating birds during these seasons. The number of migrating birds in the project area varies from day to day. Some migrating species are present every year but in relatively low numbers. Those species may, by chance, be detected in some years and not in others. An increase in winter sparrow and towhee diversity was noted and thought to be due to their use of annual forbs and grasses which responded quickly to removal of cattle. Sparrow/towhee diversity increased from 5 to 9 species during the second winter (2002) and remained at 9 in the winters of 2003, 2004, and 2005. The species most often seen were White-crowned, Song, Lincoln’s, Brewer’s, Rufous-crowned and Chipping Sparrow, and Abert’s and Green-tailed Towhee. Numbers of individuals of many of these species varied quite a bit over the five winters, suggesting that these numbers may be affected by population mechanisms on northern breeding grounds, or the tendency of winter flocks to range widely over the landscape. Another winter species, the Ruby-crowned Kinglet, has increased steadily over five winters from 7 in 2001 to 26 in 2005 (maximum number seen on a given winter survey). This, again, may be due to increased tree canopy due to cattle exclusion, and to good cottonwood-willow recruitment. The presence of Yellow-billed Cuckoo in the summers of 2004 and 2005 may be a significant indication of habitat recovery at the site. Before 2004 it was detected only once before, likely a migrating bird. But in summer surveys in 2004 and 2005, several individuals were detected. While nesting status has not been firmly determined, the sightings persisted through July in both years and into at least early August this year, suggesting that they are in fact nesting. Cuckoos are a mid-story species requiring large habitat patches. As near-river restoration plantings mature, riparian vegetation width should increase, thus providing more suitable cuckoo habitat. A concern raised by the bird surveys is the large number of Brown-headed Cowbirds observed during the spring and summer surveys. A large dairy near the site provides food for these birds, and the restoration site provides an area where they may parasitize nests of other birds. There is the possibility that even though species diversity is increasing at the site, reproductive success for parasitized species may be low. We are working to find ways to research not only the avian diversity and numbers at the site but their reproductive productivity as well. Overall, the restoration project is helping to restore bird populations that were lost or greatly reduced during the last century along the lower Santa Cruz River. The positive trends and new occurrences of avian species listed as Priority Species by the Sonoran Desert Conservation Plan (Bell’s Vireo, Abert’s Towhee, Yellow-billed Cuckoo and Swainson’s Hawk) further indicate the importance of this project.

Long-Term Management

Tucson Audubon holds a 99-year right-of-entry lease with the City of Tucson for the North Simpson Site and as such has specific requirements for long term monitoring of bird populations as well as habitat conditions for other wildlife species. Long-term monitoring will help determine the overall success of plantings at the restoration site, and how they are affecting wildlife. Regular avian surveys are carried out to see whether habitat is really being enhanced in a way that increases numbers and diversity of birds. Project managers are also monitoring survivorship of plantings, changes in vegetation coverage and diversity, and changes in the river channel morphology to determine site stability over time.

Sources and Amounts of Funding

The project was funded by in-lieu mitigation moneys from Section 404 of the Clean Water Act, as well as grants from the United States Fish and Wildlife Service as well as the Arizona Water Protection Fund. The lead organization on the project is the Tucson Audubon Society, which has provided the bulk of the volunteer labor, incorporating classes for educating interested citizens in the project and in the techniques of ecological restoration.

Primary Contact

Organizational Contact