A Model of the Middle Rio Grande Valley
River of Change: Bulldozers in the Bosque
Hey, they're bulldozing the bosque! How can that be restoration?
In recent years resource managers have been using these big machines with a new goal in mind. Instead of bulldozing the forest to clear it for human use, ‘dozers are now being used to help restore the forest. In what managers and scientists like to call “bulldozer ecology,” heavy equipment is being used to clear out exotic vegetation and alter the river bank to allow flooding. One successful example of this is the Albuquerque Overbank Project.
The Albuquerque Overbank Project (AOP) is a collaborative effort with participants from the U.S. Bureau of Reclamation, University of New Mexico Department of Biology, City of Albuquerque Open Space Division, Middle Rio Grande Conservancy District, New Mexico Natural Heritage Program, U.S. Fish and Wildlife Service, State of New Mexico Environment Department, and U.S. Army Corps of Engineers. The AOP site is located on the southern end of an elevated, attached river bar on the west side of the Rio Grande, north of Rio Bravo. Because of riverbed degradation, it had received little if any overbank flooding in recent decades. This project was designed to evaluate the effectiveness of bank clearing and lowering to reestablish native woody vegetation (cottonwoods and willows) on such a site in the Albuquerque Reach of the Middle Rio Grande bosque.
Site preparation, using bulldozers and other heavy equipment, began in March 1998. It involved clearing and root plowing 4 acres (1.6 ha) of the bar’s dense Russian olive cover, then lowering over half of the cleared area by 2 feet (0.6 m) to allow for flooding during spring runoff events and summer wet periods. Trees that were cleared were removed and chipped. Material from the bank was spread over a connected (lower) sand bar south of the cleared site. Shallow channels and topographic undulations were created on the cleared bar to facilitate floodwater distribution to promote the establishment of native tree seedlings.
A number of physical and biological parameters have been monitored throughout the project. Shallow ground water wells (piezometers) were set up at various distances from the bank to see how ground water is affected by river flow, distance to the river bank and distance to the old, established bosque. Vegetation was sampled throughout the lowered bar area to monitor which herbaceous and woody species came onto the cleared site. Soil salinity was measured and soil textural types identified. A fenced weather station was erected at the site’s north end. Surface-active arthropods, bird populations, and beaver activity were monitored on the new bar as well as in the adjacent cottonwood bosque and in an uncleared Russian olive thicket north of the site.
The site flooded in May and June of 1998. Flooding occurred at flows over approximately 2,500 cubic feet per second (cfs). In 1999, there were three overbank inundations: in late May–early June, late June and early August. Relatively elevated parts of the site did not flood, even at flows approaching 5,000 cfs. Ground water levels correlated well with Rio Grande stage heights and discharge rates measured at the Albuquerque Central Avenue Bridge USGS gages. Ground water response was most rapid and had the greatest amplitude in wells nearest the bank.
Changes in river channel morphology (shape) were quite notable. Prior to restoration activities, the river channel adjacent to the site maintained uniform depth, velocity and width for variable river discharges. However, because of the extensive erosion changes in the bank profile and in site topography, the river channel is now much more variable in depth, width and velocity for variable discharges. Since bank-stabilizing Russian olives are now absent, the river has eroded 125 to 150 feet (37.5–45 m) of bank line, with river width increased by 15 percent. New bar formation has occurred downstream as the eroded bank material has been relocated.
Over 8,000 cottonwood seedlings and a smaller number of coyote willow, saltcedar and Russian olive were established during the first flood season. Most of the cottonwoods died before the second season, but the remaining patches are conspicuous in places (some were six to seven feet (1.8–2.1 m) tall in 2001) and account for more cover than do survivors of other woody species. They occur in sandy-loamy soils that characterize much of the site. Relative saline soils at the northern end of the experimental area supported large sunflower stands the first summer. These were largely replaced by sweet clover the second summer. Cockleburs and horseweed are common toward the site's southern end.
The new bar habitat greatly increased the number and diversity of animals using the site. A number of bird and arthropod species were detected on the new bar that were not present in the adjacent mature bosque. Beavers were active cutting young cottonwoods and willows, but as of the summer of 2001 had not had a large impact on the new vegetation overall. Animal use of the bar will likely change as the vegetation changes over the years, but overall the project was beneficial to animal populations.
The AOP provides a good demonstration of the type of new restoration projects currently being done along the Middle Rio Grande Valley. It also shows the effectiveness of bulldozer ecology.
