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A Snapshot, Repeated; Riparian Vegetation Change in Response to Altered Flow Regimes and Geomorphology, San Juan River, SE Utah; Cynthia Dott

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2021 Conference
A Snapshot, Repeated; Riparian Vegetation Change in Response to Altered Flow Regimes and Geomorphology, San Juan River, SE Utah
Cynthia Dott1*, Gary Gianniny2
1Department of Biology - Fort Lewis College, Durango, CO, USA;
2Department of Geosciences - Fort Lewis College, Durango, CO, USA;
Western rivers, always highly variable in terms of hydrology and sedimentation, have experienced a tremendous amount of change during the last century.  The San Juan River in the Four Corners region of North America’s southwest is no exception, and its lower reaches above Lake Powell provide an opportunity to observe how variation in geomorphic setting, sediment dynamics and hydrology – due to damming and climate change – have driven changes in woody riparian vegetation (both native and non-native).  By using a combination of repeat photography, dendrochronology and plant community data, we can begin to tell the story of sequential waves of vegetation colonization that have occurred over this century of change. 
There are several key changes we have observed in this system:
  • In the 1920s, very little vegetation existed in any portion of the river corridor, including wide alluvial reaches, due to scouring by huge floods in the 1890s, 1911, and 1927.  By the 1940s-50s cottonwood (Populus fremontii) and willow (Salix exigua) had begun to colonize portions of the alluvial floodplain, but in the bedrock canyons very little vegetation occurred other than sparse willow.
  • Beginning after the completion of Navajo Dam upstream in 1962, the braided alluvial channels began to narrow, islands stabilized with vegetation, and total abundance of riparian plants increased.  Zones of cottonwood at the rear, with tamarisk (Tamarix spp) and willow closer to the banks became established. 
  • In contrast, there is little evidence of channel narrowing or sediment accumulation in the bedrock canyons.  However, tamarisk and willow – but not cottonwood – colonized the river banks in many portions of the bedrock canyons beginning in the 1950s or 60s.
  • Tamarisk, once dominant in alluvial valleys and locally in bedrock canyons is now declining, at least in part due to the successful biocontrol of tamarisk leaf beetle (Diorhabda spp).
  • Russian olive (Eleagnus angustifolia) colonization occurred in the alluvial floodplain perhaps as early as the early 1960s (based on dendrochronology data), and they were certainly present by the early 1970s based on historic photographs.  By the 1990s Russian olive (RO) formed dense stands along the river banks in the alluvial valleys.  Since this time, RO has begun migrating downstream into the constrained bedrock canyons.  RO saplings are now observed in all reaches – regardless of geomorphic type - but can be hard to detect as young individuals because they grow up through the willow canopy, or in some cases form a low ground cover due to heavy browsing by beaver!
  • There is an astonishing increase in willow density and height in all reaches, but most notably on Lake Powell sediment that is now creating low, inset alluvial floodplains within the bedrock canyons below Slickhorn Gulch/above Clay Hills.  These areas also host Baccharis spp (seep willow), Phragmites (giant reed), and the invasive ravenna grass (Saccharum ravennae).
It is important to remember that vegetation responds to changes not only in geomorphic setting, but also to changes in hydrology and disturbance.  The pre-dam conditions that once favored cottonwood and tamarisk colonization no longer exist for most of this portion of the San Juan River.  Instead, lower dam-release flows, the lack of high flow erosion, and decreased flows associate with multi-year drought favor coyote willow and Russian olive.  Based on changes observed in repeat photography and the abundance of RO saplings in all reaches of the river, we predict that without significant removal efforts Russian olive will achieve high densities throughout the canyons within the next 10-20 years.

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