Comparison of Geomorphic Change in Two Flood Events on

the Umatilla River, Oregon

Patricia McDowell, Michael L. Hughes, and W. Andrew Marcus

Department of Geography, University of Oregon

http://geography.uoregon.edu/mcdowell/umatillasupported by NSF Geog & Reg Sci award BCS 0215291

Hypotheses and Goals

• Floods reshape channel and floodplain, create fish habitat

• Humans respond to floods with levees and bank protection

• Bank protection reduces the ability of the river to create and renew fish habitat

• Goals: Reconstruct geomorphic change in two flood events, 1950s-60s & 1990s Investigate effects of bank protection

Outline

• Flood events and study area

• Methods

• Overall changes

• Spatial patterns (by segments)

• Examples

• Future work and significance

1235

Pendleton

Columbia River

Methods• Pre- and post-flood aerial photos• Scanned and georectified in Erdas

Imagine®

• RMSE <3 m• Classification and digitizing of channel and

floodplain features in ESRI ArcMap®

• Analysis of change: amount, type and spatial distribution

Segment 1

Active channel area increases in both floods. Sinuosity decreases in both floods. Change in greater in 1965 than 1996; bank protection?

Scoured area and bar area increase in both floods.Vegetated area increases in 1965 by capture of vegetated islands

Overall Changes• Bars and scoured areas increased in both flood

events • Sinuosity decreased slightly in both flood events• 1965 flood produced more change than 1996

flood: bank protection following 1965 flood• Active channel area increased in 1965 flood, but

did not change in 1996 flood• Vegetated surfaces in active channel zone

increased in 1965: capture of new vegetated islands by avulsion

1235

Pendleton

Columbia River

Segment Geomorphology

• Segments 1 and 2: narrow valley, confined meanders, less area in bars and scoured surface,moderate sinuosity

• Segment 3: wide valley, unconfined meanders, more area in bars and scoured surface,higher sinuosity

• Segment 4: urban, not analyzed• Segment 5:

narrow valley, confined meanders, very low sinuosity

Upstream

Downstream

All segments increase in 1965 flood. 1 and 2 have no increase in 1996 flood: bank protection?

Change in Active Channel Area

Change is inconsistent across segments and flood events.Discontinuities in sediment transport and storage?

Change in Bar Area

All segments decrease in 1965.Change is inconsistent in 1996.

Change in Sinuosity

Changes by Segment

• Segment 3 (wide) is more active than 1 and 2 (narrower)– Seg 3: greatest increase in active channel

area– Seg 3: greatest decrease in sinuosity

• Segment 5 (narrow, low sinuosity): – Increases active channel area – Change in bar area, sinuosity is small

• More change in 1965 flood than in 1996

Segment 3

Segment 3

Partly confined reach

Lateral migration, meander cutoff, and decrease in sinuosity in both floods.

Segment 1

Reach is confined at upstream and downstream ends.River tries to re-establish meander in 1996 flood.

Segment 5

Most active reach of segment 5

Incised valley meander

Lateral migration and avulsion in both events

Segment 5

At upstream end of Horseshoe curve, looking downstream

Conclusions• Enlargement of active channel zone,

capture of islands • Avulsion, lateral migration• More change in 1965 than in 1996:

increase in bank protection• Decrease in sinuosity in floods, little

recovery between floods: bank protection?

• Spatial differences at segment scale, reach scale – Need to examine at finer spatial scale

Next Steps

• What specific kinds of channel change produce features important for fish habitat? – For example, how are side channels

created?– Help restoration projects mimic natural

processes

Next Steps

• What controls where the channel is active vs. inactive?– Are there natural “hot spots” for channel

change?– Avoid rip-rap, levees in key sites where

habitat potential is highest– Understand the controls, so results can be

applied to other rivers

Next Steps

• What happened in between the 1965 and 1996 floods? – Do moderate sized floods also produce

channel change?– Types and magnitude of change?– Help understand how channel features

evolve over several decades

• New rivers: Walla Walla, John Day

Next Steps• What specific kinds of channel change

produce features important for fish habitat?

• What controls where the channel is active vs. inactive?– Are there natural “hot spots” for channel

change?

• What happened in between 1965 and 1996 floods? What is role of moderate-sized flood events?

• New rivers: Walla Walla, John Day

In Horseshoe curve, looking upstream