The Western San Juan Mountains: Their Geology, Ecology, and Human History

13. Wetlands, Riparian Habitats, and Rivers

CHAPTER 13 WETLANDS, RIPARIAN HABITATS, AND RIVERS PRESTON SOMERS AND LISA FLOYD-HANNA Moisture is one of the premiere limiting factors for plants and animals ' and where moisture is not limited one finds very distinctive communities of drought-intolerant species. Wetlands, defined as areas that remain moist through all or most of the growing season, include communities such as marshes and bogs. A special rype of wetland is a riparian ecosystem (i.e., the area immediately adjacent to a flowing stream). In this chapter we discuss the characteristics and importance of the wetlands of the San Juans in general, then look more closely at the riparian vegetation that grows along the rivers and streams of the San Juan Skyway region. Finally, we discuss the aquatic ecology ofa representative major river, the Animas. WETLANDS In a region characterized by low and seasonal precipitation, the presence ofwetlands seems surprising. However, these isolated areas have a unique or persistent water source and are highly productive and diverse ecosystems, especially when set against an arid background. Often called the "thin green line," rich, wet ecosystems occur along waterways at both high and low elevations. Although wetlands make up only a fraction of the total land area in Colorado, it is estimated that 70 percent of the animals, including 80 percent of the vertebrate fauna, depend on wetlands for their permanent homes or for stops along their migration path. 175 176 The Western San juan Mountains Wetlands are disappearing all over the United States. In the last century, for example, Arizona and New Mexico have lost 90 percent oftheir original riparian and wetland ecosystems (Johnson 1989). A draft listing ofthe threatened native wildlife in Arizona compiled by the Arizona Game and Fish Department in 1988 named eighty-one vertebrate species associated with wetland habitats , ofwhich 70 percent absolutely require wetlands and riparian sites to survive. Thus, the importance of wetlands extends beyond the ecological characteristics and processes we generally associate with them - for example, speedy nutrient cycling; rich, often oxygen -deficient soils; and reduction of river sediments. Many of the western wetlands are seasonal, driven by snowmelt or other fluctuations typical of western hydrology. Sometimes , as in the wetland-rich San Luis Valley, a shallow water table provides for the characteristic wetland vegetation. In montane regions , wetland meadow vegetation is supported by a shallow water table fed by spring snowmelt. Other wetlands persist on the edges of rivers and streams. Wetland plants have many adaptations that allow them to thrive in waterlogged soils. For example, their roots can tolerate low oxygen levels. They also have a low susceptibility ro the toxins produced by microorganisms that occur in oxygen-deficient soils. Roots of wetland plants contain a spongy tissue full of large air spaces, which enhances oxygen uptake. Distinctive wetland species in southwestern Colorado include cattails, many species of sedges, willows, cottonwoods, and horsetails (Equisetum spp.). In summer wetlands buzz with dragonflies and birds such as the rough-winged swallow and redwinged blackbird. Wetlands are a threatened kind ofecosystem throughout the West and are protected by federal legislation. Many of the criteria used to define a wetland, however, are derived from the more common eastern types. Hydrological cycles, soil types, and plant species are used together to determine whether or not a site is a so-called "jurisdictional wetland" worthy ofprotection. There has been a strong lobbying effort to reduce the amount of land that falls into the wetland definition. Colorado wetlands, primarily because of their seasonal nature, may lose their protected status if Wedands, Riparian Habitats, and Rivers " I77 legislation is weakened. At present, if a site is saturated for seven to ten days, it may qualifY for wetland status. A proposed change in legislation would expand the minimum saturation time to twenty -one days. Such a change would eliminate up to 50 percent of protected sites. Wetlands appear at many places along the Skyway and throughout the San Juans. A beautiful, low-elevation wetland ecosystem can be viewed from the highway just east of Mancos. Numerous examples of high-elevation wetlands lie near Molas Pass. These support different species than the wetlands in the foothills do, but they exhibit the same kinds of ecological conditions and processes. RIPARIAN VEGETATION Major streams in the foothills and basins of the San Juan region commonly are lined with riparian woodlands dominated by cottonwoods (Populus spp.). These woodlands and associated phreatophytic vegetation (composed of plants that...