Channel restructuring can offer a variety of benefits for river and human systems. Positive outcomes of channel restructuring can include riparian habitat protection, general ecological resilience, the reestablishment of native species, and the creation of human designated desirable green space. This mix of human and physical system benefits make this an attractive option when approaching the Rio Chama between Abiquiu Dam and its confluence with the Rio Grande. Channel restructuring can help increase the ecological resilience of a system. As defined by Walker and Salt, the concept of resilience is the ability for a system to bounce back after a disturbance. The techniques described above can all improve the resilience the Rio Chama. As Dr. Kim Eichhorst from BEMP explains, a resilient riparian stretch of river has “mosaic” qualities. This means a patchwork of varying riparian ecosystems along the river. This helps the system as a whole respond to different disturbances such as drought, wildfire, and insect disease.
Below are a few examples of channel restructuring techniques. This type of work has many iterations and options for treatment. Please see Colorado State University's River Restoration Centre website for more details on other types of techniques.
Channel Bank Lowering
Bank lowering is when a river’s channel banks are mechanically lowered to create overbanking at higher river flows. This creates space for the river to flood, recruit new native growth, and helps decrease continued channelization. A bank lowering project on the Middle Rio Grande completed in partnership with BEMP and the US Bureau of Reclamation (Reclamation) has demonstrated the positive response from the system in the form of native species recruitment and flood mitigation.
AOP, 1998. Source: BEMP.
AOP, 2003. BEMP.
AOP, 1999. Source: BEMP.
AOP, 2007. BEMP.
The Albuquerque Overbank Project (AOP) provides a great example of the benefits of bank lowering. This project, which began in 1998, is comprised of exotic species removal, bank lowering, and overbank flooding in order to re-establish native Rio Grande cottonwood trees. After several years, the ultimate result from this project was a site dominated by cottonwoods but with increased vegetation patch diversity. As of 2013, the project site had ten different tree, shrub, and herbaceous patch types. This patchwork landscape lends itself to being resilient, especially in the face of climate change. If a site is too homogeneous, a single disturbance can affect the whole landscape. Patchwork vegetation is more dynamic and resistant to this type of shock. It is important to note that the AOP’s success was heavily dependant on the timing and duration of flooding. This indicates that many different types of solutions are needed to help the Rio Chama below Abiquiu. Most channel restructuring projects will need to correlate with high flow events.
Swales
Swales are similar to bank lowering but the specifically have the intent of wetland regeneration. Generally, a cut to the river channel will connect water to a lowland in the riparian area. Non-wetland species are removed and native, more marsh dependent, species are encouraged to grow.
Swale creation. Vegetation is removed and ground is lowered.
Swale after establishment of wetland species. Photo by Bosque Education Monitoring Program.
Channel Cuts
Another project that can inspire work on the Rio Chama is the Sandia Pueblo Project. Included in this project are side channel creation, non-native vegetation removal, and inlet/bay creation. Shallow side channels allow for native fish to spawn in slow moving water. Bays offer refuge for fish when the river drops, which can happen frequently depending on downstream need during irrigation season. These techniques were implemented by the Pueblo of Sandia in 2010 and 2011 using some funds from the US Bureau of Reclamation. Coverage of this project can be found here. These types of projects are often completed in the Middle Rio Grande to bolster silvery minnow populations. The MRG is the endangered species’ last natural habitat. That is not the case for the Rio Chama, however, it could be argued that increasing fish biodiversity and populations can assist in the overall resilience of the ecosystem. Dean Melinda Harm Benson of the University of Wyoming points out that these to objectives aren’t always synonymous in her paper, “Intelligent Tinkering: the Endangered Species Act and Resilience.” Benson argues that by focusing on the health of individual species, the system is managed just for that species, instead of for the ecological system as a whole. This is a crucial point that water managers will need to consider in the Rio Chama. Although there are many methods to achieve things like native-species re-establishment and fish populations, the concept of resilience demands a comprehensive and system based approach.
Los Lunas Habitat Restoration Site in January 2003. Photo by USBR, AAO-River Analysis Team.
Tributary Management
High flows are essential to recreate natural channel conditions, but due to the downstream water demands and lack of diversion structure maintenance they are not an option presently. A key component of reducing flood risk is to better manage the tributaries and the watersheds that supply the excess sediment. Limiting overgrazing and fostering healthy vegetation to keep soils in place in tributary watersheds is vital to managing erosion during large precipitation events as the area is geologically dominated by the Santa Fe formation which consists of highly erodible sands and gravel. Installing sediment control structures within the main tributaries’ channel is another viable option to alleviate flood risk and sedimentation within the Chama. Check dams are cheap and simple structures that pond flowing water. They are constructed of permeable materials (usually rocks) that control sediments by slowing flow velocity. During high flow events, which is predominantly what the ephemeral tributaries of the Rio Chama consist of, water can flow over or through the structure but slows the flow velocity enough so that the bulk of the suspended sediment is deposited above the check dam. These structures are especially effective for capturing larger particles which is precisely what the Chama struggles to move in the channel (Miller, 2000). Reducing the sediment loads brought to the river will slow the downsizing of the river. This is not a resilient solution, but it is an inexpensive and effective short-term remedy that could stall the channel’s downsizing until diversion infrastructure is reinforced and environmental flood flows are negotiated amongst stakeholders.