Wetlands occur in Arizona, although in a limited in scope compared to the other landscapes in the state. Both major and minor rivers, small lakes, marshes and other areas support a range of plant life, from small, simple grasses to large trees. Several zones exist within the wetland environment, each suited to certain types of wetland plants.
Although the vast majority of Arizona is arid desert, there are areas that have enough moisture to form wetlands, which make up approximately one percent of the state. The higher elevations of the northern part of Arizona contribute to the waters of both the Colorado and little Colorado rivers. The southern part of the state is made up primarily of the Sonoran Desert. The San Pedro, Salt, Gila and lower Colorado rivers flow through this region. These riparian areas, as well as caldera lakes, pools in rocky hollows, called tinajas, and intermittently filled lake beds, or playas, make up the available natural wetlands in Arizona.
The Toe and Bank Zones
The toe zone is the area located below the average water level for a given wetland location. A variety of plants grow in the toe zone in Arizona wetlands. Cattails (Typha), bullrushes (Scirpus), Spikerushs (Eleocharis) and various sedges (Carex) are common. These plants prefer calm waters to become well established.
The bank zone is located above the toe zone and is periodically inundated with water. Woody trees, such as Seep Willow (Baccharis) and Coyote Willow (Salix), grow well in this zone. So do grasses such as desert saltgrass (Distichlis spicata) and alkali muhly (Muhlenbergia asperfolia).
A wider variety of plants grow within the overbank zone. This region is within the flood plain and is exposed to frequent flooding every two to five years on average.
Many trees grow well in the overbank area. The Seep Willow and Coyote Willow can be found in this region, as well as other Salix species including Red, Goodding and Arroyo Willows. Cottonwoods (Populus), Arizona Sycamore (Platanus wrightii), Ash (Fraxinus velutinia), Boxelder (Acer negundo), Black Walnut (Juglans major) and several varieties of Mesquite (Prosopis) are common in the overbank zone.
Shrubs, including Mexican Elderberry (S. nigra mexicana), Quailbush (Atriplex lentiformis) and Arrow weed (Pluchea sericea) also grow in this zone.
Grasses are prolific in the overbank. Sacatons (Sporobolus), Deergrass (Muhlenbergia rigens), Western Wheatgrass (Pascopyrum smithii) and Tobosa (Pleuraphis mutica) are common.
Also referred to as the terrace or upland zone, this area lies above the flood plain and supports many types of plants. Many of the species that populate the overbank also grow in the transition zone.
Additionally, trees, such as other Prosopis species, including the Velvet and Honey Mesquite appear in the zone. Shrubs, including catclaw and whitehorn acacias (Acacia), and fourwing saltbush (Atriplex) grow here. Grasses that populate the overbank region also grow well in the transition zone. Other grasses that grow here include the Vine Mesquite (Panicum obstrusum) and Big Galleta (Pleuraphis rigida).
Wetland plants provide a means to limit the adverse effects of erosion and help to absorb excess water from intermittent flooding. The plants provide cover for wildlife and support the ecosystem by adding valuable nutrients to the soil and oxygen to the atmosphere. In addition to the naturally occurring wetlands within the state, manufactured wetlands are increasing in use as a means to enlarge wildlife habitat and to treat wastewater. Many of the wetland plants that grow naturally in Arizona are now being introduced to these areas.