Floodplain Development of the Green River in Canyonlands National Park, Utah

10/13/2016

Alex E. Walker, John C. Schmidt, Johnnie N. Moore, Julian Scott, Jonathan Friedman, Paul E. Grams, David J. Dean, Patrick B. Shafroth, David M. Rubin and David J. Topping

Department of Watershed Sciences

Grand Canyon Monitoring and Research Center Earth and Planetary Sciences Fort Collins Science Center Geosciences

1871 1914

1968 2012 Bowknot Bend

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Models of Floodplain Formation for the lower Green River

Wide channel, no floodplain growth.

Introduction of invasive tamarisk.

Significant floodplain growth, narrower channel. Hydrology Driven

Graf, 1978 Vegetation Driven

Floodplain formation and channel narrowing was observed in the 20th century concurrent with 1928 1997 declines in peak discharge. 1912

Everitt, 1978 Andrews, 1986 Allred and Schmidt, 1999

Wyoming Bowknot Bend North What is the processes and timing of floodplain formation on the lower Green River in Canyonlands National Park? What is the magnitude of narrowing? Is the river still narrowing?

Utah Fort Bottom Canyonlands

USGS 09315000, Green River, UT National Park USGS 09328500, San Rafael River, UT

USGS 09328920, Mineral Bottom, UT Bonita Bend

Colorado

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Tamarisk trees buried by floodplain deposits were analyzed using the stem‐ burial method of Friedman et al. (2005) in order to age floodplain deposits.

Canyon Wall

Stratigraphic Cross‐Section

The modern channel is 90m wide, floodplain trench is 50m long and up to 3m deep.

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floodplain trough sandy levee sandy levee

inset inset inset

inset floodplain trough

floodplain trough sandy levee sandy levee

inset inset inset

inset floodplain trough

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2006‐2015 2005‐2015 2006‐2009 Trench with dates 2003‐2006

2002‐2003 (m) 1997‐2005 datum

1991‐1996

1990‐1992 arbitrary

1988‐1991 above 1987‐1992

1986‐1988 Elevation

1985

Based on Friedman et al., 2005

Distance along trench (m)

The youngest inset floodplain formed after 1985, narrowing the channel by 10m and building vertically 1.5‐2m of floodplain.

Inundation levels for discharges, derived from the Stage‐discharge relation for trench site stage‐discharge relation (m)

datum

arbitrary

above

Elevation

Distance along trench (m)

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1985‐1986

The floodplain deposits for this time period were underwater greater than 50% of the time and represent deposition as part of the active channel (m)

above

datum

Elevation

arbitrary 1985‐1986 Pre‐1985

Distance along trench (m)

Deposition as part of the active channel

Bank‐attached bar

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1987‐1992

In this time period deposition underwent a fundamental shift from a surface that was frequently inundated to a surface that is now inundated only by episodic peak flows. (m)

above

datum

1987‐1992

Elevation 1985‐1986

arbitrary Pre‐1985

Distance along trench (m)

1993‐2004

Long duration high peak flows in the mid 1990s continued to horizontally aggrade and build floodplains. (m)

1993‐2004 above

datum 1987‐1992

Pre‐1985 1985‐1986 Elevation arbitrary

Distance along trench (m)

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2005‐2015

High peak flows continue to build the floodplain, but at a much lower rate compared to the late 1980s and early 1990s.

2005‐2015 (m)

above 1993‐2004 datum

1987‐1992

Elevation Pre‐1985 1985‐1986 arbitrary

Distance along trench (m)

1985‐2015

Approximate discharge required to inundate aggrading floodplain

2005‐2015 (m)

Vertical accretion 1993‐2004 since 1985 means a above 1987‐1992 larger flood is now datum required to inundate Pre‐1985 1985‐1986 the floodplain surface. Elevation arbitrary

Distance along trench (m)

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Average channel width 1940‐2014 160 155 150 (m) Trench 145 140 Width 135 130

Channel 125 120 115 1940 1950 1960 1970 1980 1990 2000 2010

Channel narrowing observed in the trench is repeated over a 14.5 km reach of the lower Green River (m) datum arbitrary above

Elevation Distance along trench (m)

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An episode of high peak flows followed by a period of low peak flows allowed for floodplain aggradation. High peak flows in the mid‐1990s were unable to re‐widen the channel.

Findings

• The lower Green River has narrowed by 15% since the mid‐1980s • Narrowing has occurred by the aggradation of bank attached bars and subsequent vertical accretion of floodplain deposits. • Floodplain growth is primarily caused by aggradation during snowmelt floods. • The most recent phase of floodplain formation began in 1985 during a period of small snowmelt floods. • The 3rd largest flood of record that occurred in 2011 vertically accreted the floodplain and did not cause floodplain stripping.

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Questions?

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Flow Regime

Sediment loading data?

Also shade between lines in (ft Discharge R 3 /s) Suspended Silt-and-Clay concentration (mg/L) concentration Silt-and-Clay Suspended

Mean concentration per unit of discharge (mg/L)

+/- 1 Standard deviation of mean concentration

n = 63,259