The Albuquerque Overbank Project, Albuquerque, New Mexico

Re-establishment of native vegetation to the banks of the Middle Rio Grande

The Middle Rio Grande (MRG) is a 190-mile (320 km) stretch of the Rio Grande as it flows through the state of New Mexico. The MRG was once a flood-dominated system that created a mosaic of river channels, marshes, wet meadows and riparian forests of various ages. Construction of dams and levees, installation of Keller jetty jacks, and increased urbanization within the twentieth century have dramatically altered this system.

Half of the wetlands in the drainage were lost between 1940 and 1990. Native cottonwood (Populus deltoides var. wislizenii) trees stands seem to be in decline in many sections of the river as a result of eliminated overbank flooding and lowered water tables in areas characterized by river channel incision.

These effects have been compounded by the invasion by non-native plants. Open gallery forests once dominated by native cottonwood, New Mexico olive (Forestiera neomexicana), and coyote willow (Salix exigua) have become dense stands of mixed native and non-native species, such as salt cedar (Tamarix chinensis) and Russian olive (Eleagnus angustifolia). Monotypic stands of non-native species also are common, especially lining banks along the river.

Changes in species composition and stand structure have led to more stable banks, lowered water tables during the growing season, and increased fire incidence within the riparian corridor.

The power of interdisciplinary teamwork and project monitoring

Many restoration and rehabilitation efforts have been initiated to overcome non-native species invasion, increasing fuel loads, channel narrowing and incision, and the lack of overbank flooding.

The Albuquerque Overbank Project (AOP) is one example of a collaborative restoration "experiment" between the Department of Biology and the New Mexico Natural Heritage Program at the University of New Mexico, the U.S. Bureau of Reclamation, the U.S. Fish and Wildlife Service, the Middle Rio Grande Conservancy District and the City of Albuquerque - Open Space Division.

The project was designed to evaluate the effectiveness of bank clearing and lowering to re-establish native woody vegetation (cottonwoods and willows) through the re-instatement of intermittent overbank flooding.

The AOP site is located the southern end of an elevated alternate river bar on the west bank of the Rio Grande. The site has received little, if any, overbank flooding since the mid-1970s.

Dense thickets of Russian olive dominated vegetation at the site prior to site manipulation. The Bureau of Reclamation funded the site preparation, which began in March 1998.

Preparation of the 4-acre site involved 1) clearing, root plowing, and chipping of Russian olive; and 2) lowering 2.4 acres adjacent to the river by approximately two feet and dredging of several shallow channels within the site to allow for flooding during spring snowmelt runoff events and summer wet periods.

The site flooded in May and June of 1998. Flooding occurred at flows over approximately 2,500 cfs (70 m3 s-1). Inundation again occurred in late May, late June, and early August of 1999.

Relatively elevated parts of the site did not flood, even at flows approaching 5,000 cfs (140 m3 s-1). The site was flooded once more in May 2001 at flows again nearing 2,500 cfs (70 m3 s-1). Clearly, the site design met the objective of increasing the frequency of overbank flooding at the site.

Monitoring of shallow groundwater levels, soil texture and salinity, and vegetative succession was an integral part of the restoration plan from the beginning.

A network of groundwater wells was established at the site to assess the response of groundwater table elevations with river flow and to test the relationship of vegetation type and cover with groundwater table elevation.

Groundwater levels have correlated well with stage heights and discharge rates measured at the Albuquerque Central Avenue Bridge USGS gage. Groundwater response has been most rapid and has had the greatest amplitude in wells nearest the bank.

The New Mexico Natural Heritage Program has conducted monitoring of vegetation change over time. Over 8,000 cottonwood seedlings, and smaller numbers of coyote willow, salt cedar and Russian olive, were established during the first flood season of 1998.

Most of the cottonwoods died before the second season, but the remaining patches are conspicuous in places (some are 15 – 20 feet in height) and account for more cover than do survivors of the other woody species. The cottonwoods occur in sandy-loamy soils that characterize much of the site.

Relatively saline soils at the northern end of the experimental area supported large sunflower (Helianthus annuus) stands the first summer; but these were largely replaced by sweetclover (Melilotus officinalus) the second summer. Today, the site is a mix of native and non-native herbs, grasses, shrubs and trees.

Management of non-native species continues. For example, Russian olive trees that had established after the site was cleared were cut and treated with herbicide this past summer.

The AOP project is successful in the sense that flooding has been re-established and non-native plants are once again the dominant type of vegetation at the site. The interdisciplinary team that collaborated on this project in 1998 is still intact and cooperating in the management and monitoring of the project. Similar restoration "experiments" are being planned for other areas along the Middle Rio Grande.

Case study courtesy of Jennifer Follstad Shah, University of New Mexico / National River Restoration Science Synthesis with materials provided by Nancy Umbreit of the U.S. Bureau of Reclamation, Dr. Cliff Crawford of the University of New Mexico, and Esteban Muldavin of the New Mexico Natural Heritage Program.

Posted October 21, 2003