Floating Wetlands - Project Concluded September 2003

Floating Wetlands in the Bay
The Bureau of Reclamation recently completed a floating wetland project in Las Vegas Bay at Lake Mead. The project was conducted June 2000 through September 2003.

The project goals included:

1. Develop a floating platform and islands that are structurally durable.
2. Determine how effective floating islands are at removing nutrients, and improving water quality within Las Vegas Bay at the mouth of Las Vegas Wash.
3. Evaluate the riparian vegetation growth compared to the nutrient uptake.
4. Evaluate the plant establishment success and different planting techniques.

Floating wetlands have been proven to be effective in ponds, lakes, and reservoirs of Japan. They are an innovative way to improve water quality and clarity, provide shoreline protection from waves and to enhance habitat for fish and wildlife. The installation of the floating wetland structures (view the platforms) at the Wash occurred in November 2000. The platforms were first planted in December of that year.

How Does It Work?
The floating wetlands were made up of a series of 4-foot by 4-foot plastic pallets filled with coconut fiber to support wetland vegetation such as cattail, bulrush, and sedge. A series of the vegetated pallets were attached together and secured to the floating platform superstructure. The floating platform superstructure was a large rectangular dock composed of a series of boat slips within which the vegetated pallets were inserted. The entire superstructure was secured in place by anchors from the bottom of Las Vegas Bay. Floating wetland platforms were more than 2,000-ft. sq in size.

Water Quality Polishing
Wetland vegetation, such as cattails, bulrush, and sedges, spread roots and rhizomes through the coconut fiber substrate and penetrate the water column. As with terrestrial wetland systems, the floating wetland vegetation helps polish the water by removing nutrients and other compounds from the water column. Research and monitoring programs have been established to monitor the wetlands effect on salinity, trace elements, fecal coliform, perchlorate, and cryptosporidium. Research has included monitoring aquatic invertebrates, zooplankton, and phytoplankton use of the area below the floating platforms in comparison to open water areas, and wildlife and fishery habitat evaluations both above and below the vegetated platform.

Wildlife and Fishery Habitat
An additional benefit of the floating wetland is increased habitat. By maintaining the wetland vegetation, the wetlands provide habitat for native and migratory fish and wildlife. Isolated from the shore, birds are protected from their natural terrestrial predators like coyotes and kit foxes. The water column around the wetland platform is a highly productive area that attracts fish and many other aquatic species. The floating wetlands improved this productive area by providing shade, nesting, and hiding areas for many species.

Results
The project was concluded in September 2003. Due to lowering water levels in Lake Mead and increased sedimentation in Las Vegas Bay from the Las Vegas Wash, the platforms were relocated three times during the study period. Silt deposition and loss of lake elevation became so extreme that one of the platforms became so laden with silt that it sank on May 20, 2002. Despite the many challenges that were faced during the study period, observations made at the platforms over the course of the three-year period did provide some practical information.

Water Quality
A nutrient reduction was identified under the cattail and bulrush platforms. Unfortunately, it was not possible to determine how well the floating islands removed nutrients, compared to natural wetlands because they had to be moved several times during the course of the study.

Vegetation and Establishment
At least eighteen species of plants grew on the floating islands. Common reed proved to be an aggressive weed on the platforms and competed with the cattail and bulrush species. The aerial coverage of the plants was relatively low ranging from 25 to 60% at the platforms. However, the height of the plants reached 11 feet for cattails and 8 feet for the Olney's bulrush.

Floating Island Durability
The use of floating islands on a fluctuating reservoir posed many challenges to the engineering of the structure. In areas where anchors are not silted in, the platforms can be easily moved by using the winches on the platform to lift the anchors as needed without costly equipment and contractors. Otherwise, if the structures need to be moved, it can be quite costly. Using a soft connecting system for fastening the pallets together rather than using the stainless steel hardware that was originally used could improve the durability of the structure. Removing the silt that accumulates on the floating islands proved to be an ongoing maintenance issue as well.

For more information on this project you can read the full report, Technical Memorandum No. 8220-03-09: Water Quality and Plant Growth Evaluation of the Floating islands in the Las Vegas Bay, Lake Mead, Nevada, published December 23, 2002.