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Since the early 20th century, the Green River, the longest tributary of the Colorado River, has narrowed, decreasing available riparian and aquatic habitat. Initially, the widespread establishment of non-native tamarisk was considered to be the primary driver of channel narrowing. An alternative hypothesis postulated that changes in hydrology drove narrowing. Reductions in total streamflow and changes to flow regime occurred due to wide-spread water development, decreased snowmelt flood magnitude, and the increased cyclicity of wet and dry years. The two hypotheses agree on channel narrowing, but each influences modern river management differently. A tamarisk-driven model of narrowing implies that modern flow management doesn’t substantially affect channel change. Conversely, channel narrowing driven by changes in hydrology implies that present flow management decisions matter and continued adjustments to flow regime may result in future channel change. To understand the roles of decreasing total annual flow, declining annual peak flood magnitude, and changing vegetation communities on 20th century channel narrowing, we investigated channel narrowing along the lower Green River within Canyonlands National Park (CNP). Previous studies agree that the channel has narrowed, however, the rate, timing and magnitude of documented narrowing are only partially understood. Multiple lines of evidence were used to reconstruct the history of channel narrowing in the lower Green River. This study focuses on channel narrowing, but additionally investigated possible changes to channel depth, identified process, timing and magnitude of floodplain formation. Floodplain formation was described in the field using stratigraphy, sedimentology, and dendrogeomorphology exposed in a floodplain trench. Channel and floodplain surveys were conducted to determine possible changes in bed elevation. Additionally, existing aerial imagery, hydrologic data, and sediment transport data were analyzed. These techniques were applied to determine magnitude, timing and processes of channel narrowing at multiple spatial and temporal scales. The floodplain investigation identified a new period of channel narrowing by vertical accretion after high peak flow years of 1983 and 1984. Narrowing was initiated by vertical accretion in the active channel, deposited by moderate floods exceeded more than 50% of the time. Vertical accretion continued in the early 1990s, converting the active channel into a periodically inundated floodplain surface. Suspended-sediment deposition dominated deposits, resulting in the formation of natural levees and floodplain troughs in both inset floodplains. Rates of deposition were highly variable, ranging from 0.03-0.50 m/yr. The lower Green River within Canyonlands National Park has narrowed substantially since the late 1800s, resulting in a narrower channel. Changes to flood magnitude, rate and timing since 1900, driven by increased water storage and diversion in the Green River basin and declines in annual precipitation, were responsible for inset floodplain formation. Floodplains of the contemporary lower Green River in CNP began forming in the late 1930s and continued to form and vertically aggrade in the 20th century by inset floodplain formation. During this time period, peak flow and total runoff declined due to climatic changes and water development. Analysis of aerial imagery covering 61 kilometers (km) of the Green River in CNP shows that changes to the floodplain identified in the trench are representative of the entire study area. The establishment of non-native tamarisk (Tamarix spp.) did not drive channel narrowing, though dense stands stabilized banks and likely promoted sediment deposition. The lower Green River narrowed 12% from 1940-2014, with the majority of narrowing (10% of all narrowing) occurring from the 1980s to the present. Inset floodplain formation reflects changes to flood magnitude and timing resulting from water development and decreases in natural runoff. Findings suggest that long-term management of the riverine corridor within Canyonlands National Park will require a greater focus on upstream flow contributions and how those flows are currently managed. Recovery of endangered endemic native fishes, the Colorado pikeminnow (Ptychocheilus lucius), and the razorback sucker (Xyrauchen texanus), plays a primary role in determining current flow allocations. Collaboration with upstream stakeholders and managers is necessary to maximize elements of the flow regime that preserve channel width and limit channel narrowing.

Since the early 20th century, river channels of the Colorado River basin have narrowed, decreasing available riparian and aquatic habitat. Changes are considered to be the result of three major factors: wide-spread water development, increasing hydroclimate variability and the invasion of non-native tamarisk (Tamarix spp.), altering flow regime and sediment supply. Different studies have reached different conclusions about the relative roles of flow regime, sediment supply and tamarisk in causing narrowing. I investigated channel change in the lower Green River within Canyonlands National Park to describe channel changes in the 20th century and understand the roles of shifting flow regime and changing vegetation communities on 20th century channel narrowing. The lower Green River within Canyonlands National Park has narrowed substantially since the late 1800s, resulting in narrower channel. Changes to flood magnitude, rate and timing since 1900, driven by increased water storage and diversion in the Green River basin and declines in annual precipitation, was responsible for inset floodplain formation documented in this study. I used multiple datasets to reconstruct the history of channel narrowing in the lower Green River and identify processes of floodplain formation. In the field, analyses of a floodplain trench were described to identify rate, timing and magnitude of floodplain formation. Channel and floodplain surveys were conducted to determine possible changes in bed elevation. Additionally, I analyzed existing aerial imagery, hydrologic data, and sediment transport data. I applied these techniques to determine how floodplain formation occurred at multiple spatial and temporal scales. My investigation shows that the floodplains of the contemporary lower Green River began forming in the late 1930s and continued to form in the 20th century by inset floodplain formation. During this time period, peak flow and total runoff declined due to climatic changes and human water development. Since the mid-1980s, inset floodplains continued to develop along the lower Green River since the mid-1980s, narrowing the river by an additional 9.4%. Analysis of aerial imagery shows that changes to the floodplain identified in the trench occurred throughout the 61 km of river I studied. Non-native tamarisk (Tamarix spp.) did not drive channel narrowing, though dense stands stabilized banks and likely promoted sediment deposition. Inset floodplain formation reflects changes to flooding resulting from water development and climate change. My findings have implications for the long-term management of the lower Green River and endangered endemic native fishes ���particularly the Colorado pikeminnow (Ptychocheilus lucius) and the razorback sucker (Xyrauchen texanus). Collaboration with upstream stakeholders and managers is necessary to preserve elements of the flow regime that preserve channel width and limit channel narrowing.