Shifts in the Structure and Composition of Riparian Vegetation in Response to Sediment Aggradation on the San Juan River

Abstract

Riparian vegetation is highly sensitive to variations in the hydrologic cycle seen on dammed rivers. The San Juan River is unique in that it is impacted by dams both up and down-stream. Upstream, Navajo Dam greatly reduces flows while downstream Lake Powell has decreased the river channel base level. This reduction in stream gradient has resulted in the formation of a wedge of sediment 71.1 feet thick at its deepest point. Differences in channel characteristics and river gradient influence vegetation shifts. This study was conducted to quantify the differences in river gradient, channel width, species diversity, percent cover, and height structure of riparian vegetation above and below sediment aggradation. Results indicate a marked difference between three distinct reaches correlating with the sediment aggradation. Upper sites showed greater species diversity, decreased density and an abundance of bare ground. An intermediate reach correlating strongly with the proposed zone of river sediment deposition by Gianniny et al., showed a significant switch to dense Baccharis/Phragmites. Species composition in the lowest sites was more homogenous dominated by the dense growth of willow and tamarisk. These results correlate strongly with differences in environmental conditions between the three reaches, especially related to channel width and river gradient.