Earth and Planetary Sciences ETDs

Publication Date

8-28-2012

Abstract

A guiding principle in river science maintains that channel systems evolve to convey the sediment loads and water discharges imposed upon them. Changes in sediment and water inputs may result in adjustments to channel geometry, bed texture, and related parameters. Over the last century, geomorphic processes along the Middle Rio Grande and one of its major tributaries, the Rio Chama, NM, have been altered by intensified land and water management and climate change. Using a GIS, channel characteristics were digitized from georeferenced photographs and analyzed, with particular attention to quantifying measurement error. Along the Rio Grande, average channel widths decreased from 516±67m to 176±7m between 1918 and 1963, mostly due to decreasing peak flows and the implementation of flood control and other engineering measures. From 1985 to 2008, widths decreased from 176 ± 23 m to 146 ± 5 m, primarily over periods of low peak flow. The Rio Chama, downstream of El Vado Dam, narrowed from an average width of 58 m to 44 m, with most of the adjustment occurring after dam closure in 1935. vi Along both rivers, evidence suggested that the spatial patterns of planform change were partly controlled by tributaries confluences. To examine tributary controls along the Rio Chama, elevation and bed sediment data were collected at 200 cross sections situated up and downstream of 26 tributary confluences along a 17 km reach situated just upstream of Abiquiu Reservoir. Compared to reaches between junctions, confluences reduced gradients and bed sediment size upstream of confluences and increased them downstream. These shifts in gradient and bed texture appear to drive variations in sediment entrainment and transport capacity and the relative storage of sand along the channel bed, as well. Although the larger clasts downstream of junctions are harder to move and slow transport, the steeper slopes at these location likely help pass smaller gravel and sands delivered by the tributaries. However, channel form and process are highly variable along the study reach, reflecting variations in the sediment inputs related to watershed geology, mainstem morphology, and past depositional events.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Coonrod, Julie

Second Committee Member

Turner, Thomas

Third Committee Member

Pitlick, John

Project Sponsors

United States Army Corps of Engineers, New Mexico Geological Society

Language

English

Keywords

Fluvial Geomorphology, Dams, Tributaries, Sediment Transport, Aerial Photography Error

Document Type

Dissertation

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