Lessons Learned for Riparian Habitat Restoration Along the Rio Grande in Southern New Mexico and West Texas
Elizabeth Verdeccia1
1International Boundary and Water Commission, US Section, Environmental Management Division, El Paso, TX, USA; elizabeth.verdecchia@ibwc.gov
The U.S. Section of International Boundary and Water Commission (USIBWC) shares lessons learned from a ten-year implementation of riparian habitat restoration on nearly two dozen sites throughout the Rio Grande Canalization Project (RGCP) in southern New Mexico and west Texas.
In 2009, USIBWC signed the Record of Decision (ROD) on River Management Alternatives of the RGCP, which committed the USIBWC to implement environmental measures for long-term river management of the RGCP, including restoring 553 acres of riparian habitat and developing an environmental water program.
Conditions along the arid RGCP are challenging for restoration. The scarce water is held back at dams until irrigation season begins. During the non-irrigation season, shallow groundwater drops substantially (at some sites groundwater levels have dropped nearly 13 feet), rising again only when the river is turned on again at the (unpredictable and variable) start of irrigation, and the season is increasingly shorter. Despite the harsh conditions, the USIBWC partnered with the U.S. Fish and Wildlife Service and environmental contractors to restore 22 sites. Work included saltcedar removal, earthwork, planting of native trees and shrubs, construction of irrigation infrastructure and shallow groundwater wells, and monitoring activities. Over the 10-year project implementation, the USIBWC planted over 110,000 trees and over 11,000 shrubs at restoration sites (and 36,000 more trees and nearly 1,000 shrubs at additional mitigation areas along the river). Earthwork included site grading, creation of terraces and inset floodplains, and excavation of swales and depressions in the floodplain to improve drainage and groundwater conditions near plantings. Tree planting methods included augering holes for poles or tall pots, excavating trenches for poles, and transplanting willows with root balls and topsoil intact into trenches dug down to groundwater.
Implementing such projects has led to a trove of lessons learned. Monitoring wells showed that trenches and augered holes should be at least ten feet deep for trees to survive groundwater fluctuations. When planting cottonwood or willow poles in augered holes, success was greatest when holes were completely backfilled with minimal air pockets. Cottonwoods did better in sandy soils than clay. Success varied for poles planted in trenches, likely dependent on effective backfilling. When trenched poles survived, tree density was higher than for auger methods. Trench planting willows harvested with root balls had near 100% success and resulted in faster growth and higher density and coverage than all other methods. Shrubs grown in specially-grown extra tall pots (about two feet deep) had higher success rates than standard tall pots (about 1 feet deep). Shrubs with a bowl excavated around its base also survived better. Trees and shrubs planted in excavated swales or terraces typically had better success. An inset floodplain experienced a high-flow event that wiped out many young trees during monsoon season. USIBWC also learned lessons for dealing with illicit all-terrain vehicle and motor-cross use in the floodplain.
Five of USIBWC’s restoration sites have been irrigated with surface water. All of the irrigated sites are exhibiting greater success of plantings in the areas where irrigation water has been applied. Certain irrigation techniques have worked better, including creating irrigation cells and using large PVC pipes for directing water against gravity, via pressure.
USIBWC continues to conduct monitoring of the success of the habitat sites and adaptively manage for sites’ success. Some sites have begun to develop habitat that could eventually support endangered southwest willow flycatchers or the threatened yellow billed cuckoo.