State of the sediment: symposium season

annotated region overview

One component of my graduate program in the Department of Watershed Sciences at Utah State University is to present at our annual graduate student symposium. The goal of this symposium is to present our research questions and preliminary work and solicit feedback from our interdisciplinary department. If you happen to be in Logan, Utah on April 13, 2018, come take a listen! Otherwise, you can read the abstract below. If you’ve seen some of my other research updates, this will be familiar, but with more detail and nuance.

I presented some of this research on April 1, 2018 at the Guides Training Seminar in Marble Canyon, Arizona. This event is put on by Grand Canyon River Guides, a nonprofit dedicated to supporting the Grand Canyon guiding community. That presentation was tailored toward providing guides with a working knowledge of current fluvial geomorphological process in Lake Powell and Lake Mead.


Abstract

Influences of changing base level and episodic flash floods on tributary sediment remobilization in Lake Powell

Lake Powell is the second-largest reservoir in the United States and traps all incoming fine sediment the Colorado River used to transport through Glen Canyon and into Marble Canyon and Grand Canyon. A warming climate is projected to yield less streamflow in the Colorado River and its major headwater tributaries, inevitably leading to persistently less water stored in mainstem reservoirs. Sediment transported into the reservoir by the Colorado River, San Juan River, regional rivers such as the Escalante and Dirty Devil, and small ephemeral streams now forms deltas that partially fill upstream sections of many reservoir arms, including the side canyons for which Glen Canyon was once acclaimed. During periods of low reservoir levels, this fine sediment has potential to be eroded, remobilized, and transported further into Lake Powell. Although bathymetric measurements of sediment accumulation have been made in the largest reservoir arms, little is understood about fine sediment accumulation and evacuation in scenic, smaller side canyons. We seek to understand 1) how much fine sediment is in each tributary, 2)  dominant factors controlling accumulation of fine sediment in reservoir arms, and 3) mechanisms controlling fine sediment remobilization during low reservoir elevations.

We distinguish four potential zones of Lake Powell whose tributary canyons potentially respond differently to fluctuating reservoir levels: 1) upstream from bedrock ledges that perch reservoir arms at artificially high levels, even when reservoir storage is low, 2) immediately downstream from these ledges, 3) in the central region where side canyons are significantly inundated, and 4) very near Glen Canyon Dam. We characterize tributaries by watershed area, bedrock type, and susceptibility to monsoon flash floods. We hypothesize the magnitude of sediment accumulation and evacuation differs through these four zones. We are using aerial imagery to quantify the amount of sediment accumulation in each tributary. In the future, we will measure how much sediment has accumulated in tributary mouths using aerial LiDAR and bathymetric data. We will couple this with attributes of monsoon season circulation. Ultimately, we hope to quantify the magnitude of side canyon inundation with water and fine sediment and understand potential recovery of these side canyons in the event of protracted reservoir drawdown.

 

 

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