Document Type

Dissertation/Thesis

Date Created

8-2023

Abstract

Rivers and their riparian zones provide important ecosystem services and are vulnerable to invasions from non-native vegetation that can impact community structure and river processes. Western United States rivers have been invaded by Russian olive (Elaeagnus angustifolia) and tamarisk (Tamarix spp.). The effects of Russian olive and tamarisk have been extensively studied on arid and semiarid rivers, but there has been a lack of studies on West-Central Semiarid Prairie rivers. To study the effects Russian olive and tamarisk, have had on West-Central Semiarid Prairie rivers, I used the Powder River as a model system. The Powder River is a West-Central Semiarid Prairie river. Vegetation surveys were conducted to study the spatial distribution of Russian olive, tamarisk, and cottonwood on the Powder River. Vegetation functional traits were collected to study how Russian olive might affect hydraulics compared to tamarisk and cottonwood. Topographic surveys were conducted to study how the geomorphology of the Powder River has changed in the past 42 years, and if Russian olive might have influenced that change. Tree rings from Russian olive were collected to study how long Russian olive has been established on the Powder River, how it spread, and what environmental factors affected its spread. Vegetation surveys showed that tamarisk was not abundant in the study reach. Russian olive accounted for 39% of the woody vegetation on the Powder River whereas cottonwood accounted for 59% of the woody vegetation. Russian olive was distributed over a wide range of distances from the channel center, but half of the Russian olive was located from 42.8 to 124.5 m from the channel center. More than half of the Russian olive was located below the average elevation of the channel. This implies that Russian olive is most likely the first obstacle for water, and that it will most likely be inundated during periods of high flow. Russian olive was also found to be more rigid compared to tamarisk and cottonwood, suggesting that Russian olive is more likely to obstruct the flow of water, slowing down water velocities and increasing sediment deposition. Plots with Russian olive as a dominant species had a higher vegetation roughness than plots with tamarisk and cottonwood as dominant species. This suggested that Russian-olive-dominated plots would have higher flow resistance, likely resulting in increased channel narrowing and sediment deposition. Topographic surveys showed that cross-sections with Russian olive present had increased channel narrowing, cutbank migration, and sediment deposition on bars compared to plots without vegetation, or plots dominated by cottonwood. Russian olive’s closeness to the channel, high rigidity, and roughness may be the cause of the geomorphic change of the Powder River. Tree ring records show that Russian olive has been present on the Powder River since 1972, and that its establishment increased during the drought of 2000-2010 that occurred in the area. Correlation between tree ring width and log annual discharge suggests that Russian olive obtained water from sources other than surface water. Higher temperatures in the months of December, May and June may be limiting tree ring growth whereas higher precipitation in the months of December and June may be increasing ring growth. These results suggests that Russian olive may be changing river geomorphology on the Powder River, that droughts helped the spread of Russian olive, and that specific climatic factors like high precipitation may additionally cause future spread and associated changes in community structure, hydraulics, and geomorphic changes.

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