Date of Award:

5-2007

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee

John C. Schmidt

Committee

Joel L. Pederson

Committee

Michael L. Scott

Abstract

Channel narrowing on the Green River in Utah and Colorado has been well documented by several authors and has been attributed to reductions in flow after 1930, the construction of Flaming Gorge Dam (FGD), and the invasion of the woody riparian plant Tamarisk (tamarix ramosissima) . Narrowing has occurred through the deposition of inset floodplains, which have vertically accreted within a previously larger active channel. Prior to closure of FGD, lower magnitude floods aggraded surfaces in the areas of the channel that had the highest divergence in the velocity flow field (i.e. bars and banks). These surfaces later became the platforms for deposition of large volumes of sediment by lower frequency, big floods. FGD began regulation immediately after a big flood, which had vertically accreted active bars within the channel and near the banks. The large reduction in annual peak flows after closure of FGD abandoned these surfaces, causing them to shift, nearly instantaneously, from active channel to floodplain. The process of floodplain building after the closure of FGD was similar to that of the pre-dam, but large floods became more infrequent, allowing lower elevation floodplains to aggrade within the pre-dam channel. These surfaces were vegetated and stable when bypass flooding during the 1980's deposited up to 90 cm of sediment, raising the elevation of the floodplains beyond the post-dam active hydrologic environment.

The reduction in active channel width from floodplain building has reduced in-channel aquatic habitat, while providing substrate for the establishment of Tamarisk, which has proliferated in the regulated Green River. Controlled flooding is a potential remediation strategy to reactivate stabilized channel features, and control the spread of Tamarisk, but managed flooding is expensive, political, and its potential success is unknown. Large floods have occurred in the post-dam era as hydrologic emergencies and, although they are not planned, the adjustments to the channel during these floods are a proxy for the potential of controlled floods to rehabilitate the Green River. Channel adjustments have been monitored at permanent cross sections in Browns Park and Lodore Canyon since 1994. We use 12 years of monitoring data, which includes four bypass floods, to calculate metrics of fine sediment storage at 36 locations along the river. We combine metrics at each site using a weighting procedure to estimate the general pattern and trend of channel adjustments in these reaches since 1994. These metrics are compared to similar metrics published for the Colorado River in Grand Canyon, a river with similar geologic controls and similar management challenges, but a different sediment supply condition.

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