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Abstract. A complete listing of the vascular flora of the San Juan River watershed is given. In total, 2322 species, 16 hybrid species, 39 subspecies and 188 varieties (2565 unique biological entities) are known to occur in this 65,382 square kilometer area which is centered on the Four Corners of Arizona, Colorado, New Mexico, and Utah.

            The Four Corners Region of Arizona, Colorado, New Mexico and Utah is a land of stark contrasts. As defined by the watershed of the San Juan River (Figs. 1 and 2), this region includes the high, tundra-topped peaks of the San Juan Mountains, the ponderosa pine forests of the Chuska Mountains, the sagebrush and blackbrush flats of the lowlands, the sparse vegetation of slickrock rims and canyons, oases of cottonwoods hugging small sources of perennial water, and naked dunes of shifting sands. It is a land of kaleidoscopic colors where, at first glance, geology often dominates biology. Blood red sandstones hide cool alcoves with hanging gardens and dripping walls. Black volcanic cliffs house ferns in their recesses. Pink sand dunes, upon closer inspection, are dotted with widely spaced plants. Mesas that appear to be solid tan or pink sandstone from a distance are found to be clothed in piñon pines. Ribbons of life follow watercourses carved into rock. Except in the higher mountains, where water is more abundant, plants are usually widely spaced. Although the region is dominated by geology and aridity, one biological attribute is not in short supply: diversity. This report is an attempt to document, for vascular plants, that diversity.

            Several major, relatively recent floristic treatments exist that cover the study area in a piecemeal fashion. These include A Utah Flora (Welsh, et al., 2003), Colorado Flora: Western Slope (Weber & Wittman, 2001), Manual of the Plants of Colorado (Harrington, 1964), Flora of New Mexico (Martin & Hutchins, 1980), Arizona Flora (Kearney & Peebles, 1960), Seed Plants of Northern Arizona (McDougall, 1973), and the Intermountain Flora series of publications (Barneby 1989; Cronquist 1994; Cronquist, et al 1972, 1977, 1984, 1997). In addition to these floras, family treatments for a new flora of Arizona are being published serially in the Journal of the Arizona-Nevada Academy of Science (vol. 26, 1992; vol. 27, 1994; vol. 29, 1995; vol. 30, 1998; vol. 32, 1999; vol. 33, 2001). Aquatic and Wetland Plants of Southwestern United States (Correll & Correll, 1972) is especially useful for wetland plants. A partial flora of the San Juan Mountains is also available (Komarek, 1994).

            Given that the above treatments are available, why produce a new catalogue of vascular plants? Succinctly, the San Juan Drainage is botanically under-explored. It is far from the major university herbaria, and consequently has not received the attention afforded areas more easily visited by professional collectors. (The flora of Mesa Verde National Park is a notable exception.) Much of the study area is within Indian Reservations, a political reality that has further limited collection. Much of the area is only accessible by foot or, in some cases, by boat. Most of the area is distant from roads. Our own botanical explorations have unearthed new state and county records on a nearly daily basis. Species new to science have also been frequently encountered. A second reason to compile this flora was the desire to inventory a large non-politically bounded region. We have, therefore, compiled a list of all known vascular plants within the watershed of a major western American river, from its sources high in the San Juan Mountains of Colorado to its confluence with the Colorado River (Lake Powell) in Utah. We plan on ultimately publishing a complete flora with keys, descriptions, synonymy and illustrations. It is our hope that this catalogue will stimulate others to supply additions and corrections. No flora is ever complete.