This update was originally disseminated on the Glen Canyon Institute mailing list.
Lake Powell traps all fine sediment that once moved through Glen Canyon and into Marble and Grand Canyons. Sediment transported into Lake Powell by the Colorado River near Hite, by the San Juan River downstream from Mexican Hat, and by other tributaries now forms deltas that partially fill the upstream sections of many reservoir arms. Bathymetric measurements of sediment accumulation have been made in many of the large reservoir arms, but little is understood about fine sediment accumulation and evacuation in smaller side canyons. These side canyons are the prestige of the region, one of the attributes for which Glen Canyon is known.
Projections of future runoff in the Colorado River Basin suggest there will be less water available for storage in Lakes Mead and Powell. There is longstanding debate about how much water should be stored in each reservoir. During times of reservoir low stand, fine sediment in tributary mouths can be eroded, remobilized, and transported further into Lake Powell. We seek to understand the existing patterns of sediment remobilization in small tributaries and slot canyons. By quantifying how much sediment fills each tributary and how much has been remobilized during previous reservoir low stands, we seek to better understand the restoration and management potential of these canyons.
We are just beginning our studies. We have delineated the watershed boundaries of all tributaries that drain into Lake Powell. Most tributaries are composed of Navajo sandstone, the Kayenta formation, and Wingate sandstone. This spring and summer, we will measure how much of each tributary mouth is filled with fine sediment. We will examine in the field and with aerial imagery how previously accumulated sediment above minimum power pool elevation has been remobilized and reworked by flash floods. We seek to understand how quickly and to what degree fine sediment might be removed from parts of tributaries whenever Lake Powell is low. Because there are so many tributaries, we will implement a stratified random sampling framework to study tributaries formed in different rock types. Ultimately, we hope to understand the degree to which Glen Canyon’s side canyons can be restored.