Nancy Umbreit, USBOR

Esteban Muldavin and Sarah Wood (NM Natural Heritage Program)

Ondrea Linderoth-Hummel (City of Albuquerque Open Space)

Sterling Grogan and David Gensler (Middle Rio Grande Conservancy District)

    The AOP site is located on the southern end of an elevated alternate river bar on the west side of the Rio Grande, north of Rio Bravo. Because of river bed degradation, it has received little if any overbank flooding in recent decades. This collaborative 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, funded by the Bureau of Reclamation, began in March 1998. It involved clearing and root plowing the bar’s dense Russian olive cover, then lowering part of the bar to allow for flooding during spring runoff events and summer wet periods. Piled trees were removed and chipped by Albuquerque Open Space. Of a total of 4 cleared acres, 2.4 acres adjacent to the river bank were lowered by approximately 2 feet. Eight thousand cubic yards of removed material were spread over a connected (lower) sandbar south of the cleared site, and then blended. Shallow channels and topographic undulations were created on the cleared bar to facilitate floodwater distribution leading to the establishment of native tree seedlings. Four equidistant lines, of 4 to 5 evenly spaced shallow wells (piezometers), were set up to test for the relationship of distance from bank and uncleared bosque cover to river flow regime. A grid system was initially spread over the entire experimental area. In 1999, the grid was replaced by transects that roughly covered the original east-west lines. Soil sites salinities were measured over much of the site and soil textural types identified. A fenced weather station was erected at the site’s north end.

    The site flooded in May and June, 1998. Flooding occurred at flows over approximately 2,500 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. Groundwater levels correlated well with stage heights and discharge rates measured at the Albuquerque Central Avenue Bridge USGS gauge. Groundwater response was most rapid and had the greatest amplitude in wells nearest the bank. Prior to restoration activities, the river channel, adjacent to the site, had uniform depth, velocity, and width for variable discharges. However, because of extensive erosional changes in the bank profile and change in site topography, the river channel is now much more variable in depth, width, and velocity for variable discharges.

    Over 8,000 cottonwood seedlings, and smaller numbers 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 are 6-7 feet in height) and account for more cover than do survivors of the other woody species. They 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 stands the first summer; these were largely replaced by sweetclover the second summer. Cockleburs and horseweed are common toward the site’s southern end.