Longitudinal Variation in the Vegetation Community on the San Juan River

In this paper, we analyze the longitudinal changes in vegetation composition along a
76-mile stretch of the San Juan River in southern Utah. The San Juan River woody canopy
community is dominated by two primary invasive species competing with two primary native
species. The two primary invasive species, Russian olive (Elaeagnus angustifolia) and tamarisk
(Tamarix spp.), have flourished in the San Juan River, perhaps due to the altered hydrological
regime from management of Navajo Dam (1962) and the sediment accumulation upstream of
Glen Canyon Dam, which have placed the native willow (Salix exigua) and cottonwood (Populus
fremontii) at a competitive disadvantage. In this study we conducted line-intercept vegetation
surveys in mid-June 2022 at four separate locations along the San Juan river in an effort to
document changes in the vegetation composition moving downstream along the five sites
sampled from Sand Island to Oljeto Wash. We suggest that shifts from coyote willow to
seepwillow and tamarisk reflects sediment accumulation from Glen Canyon dam transforming
the downstream sites’ floodplains into sandy ones, and that invasion of the primary nonnative
species at the upstream sites are more reflective of historic establishments persistent with
hydrological conditions set by Navajo Dam.