Wetlands Research

​​Water Quality

The constructed wetlands provide an exceptionally efficient cycle in the process of cleaning wastewater. In wetlands, natural, physical, chemical, and biological mechanisms work together to remove and transform pollutants into harmless by-products. Nutrients and toxic compounds are physically removed or transformed by bacteria residing on the bottom of the ponds and on plant surfaces. Aquatic plants produce oxygen, which helps kill bacteria and pathogens.

How Wetlands Work

List of books about Wetlands

Vegetation Sustainability

After a few years of outstanding growth, the wetlands vegetation began to senesce, or die off. This is a natural process that happens in natural wetlands. However, new vegetation was not successfully establishing within the wetland cells. Research has focused on why this happened and how to prevent its reoccurrence.
In May 2004 wetland managers, with help from the Phoenix Fire Department, conducted a controlled burn of one of the original Tres Rios wetlands research sites. As defined by fire management procedures, prescribed burning is the intentional application of fire to wildland fuels (in either their natural or modified state) under specified environmental conditions. In general, there are three classes of prescribed burns in wetlands: 1) surface/cover burns, 2) root burns, and 3) peat burns. Surface/cover burns are typically cool fires used to remove organic material. Root burns are much hotter and actually kill the roots. These are used to select for desirable species and eradicate, at least for some time period after the urn, undesirable species. Peat burns are used to establish open-water areas in large peat bogs. The research basin underwent a surface/cover burn. The goal of this prescribed burn was clear and two-fold. The main objective was to perform efficient and effective vegetation removal in the basin. This would accomplish the second objective of enhancing vector control. The additional benefits from such prescribed burns to riparian areas and wetlands are numerous. The following list is a short summary:

  • Provides important inputs (debris and litter) to the water body
  • Retains nutrients, sediment, and energy in the system
  • Releases plant nutrients following the burn

  • Makes new green shoots, roots, and rhizomes of grasses available to waterfowl

  • Exposes fallen seed for waterfowl.


Another research basin was reconfigured in the spring of 2003 to include hummocks. Hummocks are small contoured mounds that provide a gradient of water depths to control vegetation and increase the hydraulic mixing. Subsequently, by design, hummocks allow open access for mosquito control measures. The basin since then has been very closely monitored to see how depth, species selection, water chemistry, soil chemistry, and various other factors influence vegetation growth. A team of scientists from the Bureau of Reclamation, United States Department of Agriculture, U.S. Geological Survey, and Wass Gerke & Associates have been conducting this research.

Mosquito Control

Tres Rios project managers have implemented several measures since 1998 to minimize the numbers of mosquitoes breeding within the wetland as well as adults caught adjacent to the wetlands or in contributing areas. Vegetation management efforts such as the controlled burn and the basin reconfiguration have allowed for less dense vegetation which provides access to the two key larval controls, larvivorous fish and application of larvicides. Another technique has been the improved larvicide application techniques. Reductions in adult and larval counts can also be attributed to offsite treatments of potential breeding areas, and natural fluctuations in mosquito populations.

Presentation about Vector Control in the Tres Rios Wetlands.

Non-lethal Beaver Control

The Tres Rios project managers were also excited and surprised to discover soon after completion of the research wetlands that beavers, native to Arizona, had moved in once more. The arrival of the beaver was a sign that the wetlands were doing their job in attracting native wildlife and providing habitat that has been severely degraded in the last 100 years. Then, the beavers multiplied. Using a nighttime spotlight survey, beaver numbers were estimated at somewhere between 34 and 50 individuals. Other evidence of excessive beaver activity included extensive damage to wetland plants from foraging, cut or girdled trees, burrows, and runways. The outlet weirs were repeatedly clogged, making it impossible to regulate the flow and level of the water surface. Berms and maintenance roads were compromised. Water short-circuited through the runways established by beavers dragging wood to burrow sites. The detention time in the wetlands was shortened, and wetlands staff was having trouble just keeping up with maintenance support.

After consultation with the United States Department of Agriculture Wildlife Services, a cooperative non-lethal beaver research program was established. The first phase involved frightening or excluding the beavers. This didn't work very well, but there did appear to be some value in fencing off particularly vulnerable areas. The second phase developed a mobile laboratory, and performed controlled experiments with anesthetics and transmitters. Currently, beavers are being trapped and tagged for ecology and movement studies. The goal of the research is to provide a more stable beaver population, which will protect the wetland facilities while still providing habitat for this important Arizona native animal.

Beaver Research Paper

Tres Rios Beavers

Wildlife Diversity

Along with purification of water, wetlands also provide a suitable habitat for waterfowl, mammals, amphibians, and insects. Surveys have been performed by biologists to quantify the use of the wetlands by species of all sorts. The objective of the research has been to identify wildlife using the wetlands on a (1) migratory basis, (2) seasonal basis, (3) permanent (year round) basis, and (4) what species of birds are nesting at the wetlands. The surveys have been performed by various groups, most recently the United States Department of Agriculture Wildlife Services.View the Birders Page to see the avian species that frequent the wetlands.