by Sari Krosinsky

Man-made interventions like dams disrupt a river’s natural flow. Restoration strategies aim to return the delivery of water to reflect its natural pattern and course. While in some areas it is desirable to prevent erosion, in others it’s better to encourage erosion to allow the flooding necessary to support some native vegetation, like cottonwoods.

The U.S. Army Corps of Engineers conducts river restoration projects locally to encourage native vegetation and control erosion and flooding. And, in a collaborative effort, researchers in UNM’s Departments of Civil Engineering, Biology, and Earth and Planetary Sciences are studying the dynamics of river ecosystems in New Mexico to help effectively direct the corps’ restoration efforts.

Civil Engineering Professors Julie Coonrod and John Stormont and Earth and Planetary Sciences Professor Tim Wawrzyniec and Research Scientist Jed Frechette are investigating riverbank erosion. A better understanding of bank erosion can help predict how different river restoration strategies will work.

The researchers are using multiple methods to monitor bank stability. One method, Light Detection and Ranging, or LiDAR, is a laser imaging technique capable of resolution up to 1 millimeter. Another method involves placing erosion pins in sets above and below water surfaces. Erosion is determined by measuring how much of the pin is exposed over time. Additionally, the researchers are using a submerged jet apparatus to measure how much different soil types erode.

Stormont says LiDAR gives a more detailed picture and provides more data than the erosion pin method, but it creates a huge data analysis challenge. Periodic LiDAR scans from the same position reveal changes in the riverbank geometry over time, measuring migration as small as a centimeter. Scans are being conducted at five locations around Albuquerque where there has been human intervention. Coonrod says that for the most part, banks around Albuquerque are relatively stable unless people have done something to change them, like removing vegetation as is done with some restoration projects.

At the Calabacillas Arroyo site, LiDAR images taken before, shortly after, and a year after a storm on July 31, 2006, show that during the storm the arroyo dumped sediment into the river, leading to a buildup which halved the distance between banks, forcing the water to flow harder and faster. Over time, the sediment buildup has shifted downstream.

Another site in the study is the Albuquerque/Bernalillo County Water Utility Authority Dam in the North Valley. The dam, which will divert part of the river for tap water, remains below the surface until it’s inflated. Early results during the trial phase show that when the gates were lowered on the west side, the banks on that side were eroded by about half a meter.

To further look at changes along riverbanks and islands, Earth and Planetary Sciences Research Assistant Ben Swanson and Professor Grant Meyer have done aerial photographic analysis combined with the hydrologic record. Swanson is able to document geomorphic changes in response to both anthropogenic and hydrologic events. Combining the geomorphic and erosion studies allows for identifying target areas for restoration or vegetation management projects.

Stormont, Coonrod, and Biology Research Professor Emeritus Cliff Crawford are studying how the diversion dam will affect groundwater-surface water interactions and the health of riparian (riverside) vegetation. Civil Engineering Graduate Assistant Kelly Issacson is developing a model to predict groundwater level as a function of flow rate, to help sustainability management in different areas of the Bosque.

The flow of river water is connected with the depth of groundwater, which is in turn connected with vegetation on the banks. Cottonwoods, for example, need the water table to be no deeper than about three meters, or approximately 10 feet, for most of the year in order get enough water to survive.

Groundwater monitoring wells bracket the diversion dam at four sites, directly upstream, and downstream of the dam and on each side of the river. Measurements were taken before the dam was built, during construction, and during trial operations to determine how the dam influences groundwater levels. Biology Research Scientist Christian LeJeune records and analyzes these ongoing groundwater measurements.

The groundwater levels respond almost instantaneously to the river level. As river levels are raised upstream of the dam, groundwater levels are also raised. Similarly, downstream of the dam, the river levels and groundwater levels are lowered. It’s still too early in the research to determine how much the dam’s operation and corresponding changes in groundwater will effect Bosque vegetation.

Another aspect of the research looks at soil properties and soil moisture. The soil is made up of layers ranging from sand to clay, with layer composition varying greatly, even among different samples from the same well site.

The researchers are also measuring leaf and woody litter fall and vegetation cover as indicators of ecological health of the Bosque.

The monitoring wells serve as educational as well as research sites. Elementary, middle, and high school students participating in the Bosque Ecosystem Monitoring Program (BEMP) “track aspects of environmental change that we think are fundamental to the functioning and health of the Bosque ecosystem” under the supervision of teachers and UNM student interns, Crawford says. BEMP, founded in 1997, is a partnership between the UNM Department of Biology and the Bosque School, a private school active in Bosque studies.

A new BEMP site was added at the Albuquerque BioPark wetlands about a year ago, where researchers are studying the groundwater dynamics between the riparian forest, the river, a pond, and a marsh. The research could reveal ways to recreate wetlands along other parts of the Bosque.

Biology Professor Cliff Dahm, Research Scientist Jim Thibault, and Research Assistant Professors James Cleverly and Kristin Vanderbilt are mapping the amount of water released into the air from the plants and soil along the river, a process called evapotranspiration. As part of the research they are exploring the effects of a wildfire in Albuquerque’s South Valley during a drought in 2006.

The researchers have been monitoring the site since 2000. The human-caused wildfire burned the area south of the monitoring tower, providing an opportunity to study the fire’s effects on evapotranspiration.

When the study began, the vegetation was dominated by dense native cottonwood forest with a nonnative saltcedar and Russian olive understory. A restoration project was conducted from 2002 through 2004 to remove non-native vegetation in the hope of reducing water use and fire risk. Evapotranspiration decreased after the restoration, but increased again as non-native plants regrew. Cleverly says that damage from the 2006 fire would probably have been much greater before the restoration.

After the fire, evapotranspiration decreased and temperature increased. Annual plants grew thickly in the sunny spots left between the burnt trees, sucking moisture from the soil.

Coonrod says the data collected from these and other projects will help determine the best sites and methods for restoration efforts.