Event Title

The Changing Geomorphic Template of Native Fish Habitat of the Lower San Rafael River, Utah

Location

Eccles Conference Center

Event Website

http://water.usu.edu/

Start Date

3-30-2011 1:40 PM

End Date

3-30-2011 2:00 PM

Description

The physical template of the aquatic ecosystem of the lower San Rafael River changed drastically during the 20th century. Today, the lower San Rafael River still provides patches of complex habitat for roundtail cnub, Gila robusta robusta, flannel mouth sucker, Catostomus latipinnis, and bluehead sucker, Catostomus discobolus. Early 20th century photos show a wide, laterally unstable channel and a floodplain that is relatively low and was probably frequently inundated by overbank flows. 1938 aerial photographs also depict a complex, multiple thread, baseflow channel with numerous bars. During the subsequent 60 years, the channel simplified into a single-thread with low width-to-depth ratio. As a result of narrowing, the current channel contains few riffles and is only connected to the floodplain during rare, large floods. Aquatic habitat complexity is reduced. Three factors caused these changes in channel geomorphology: (1) reduced magnitude and duration of the spring snowmelt flood, (2) dense establishment of tamarisk (Tamarix spp) throughout the alluvial valley, and (3) continued supply of fine sediment from ephemeral tributaries. We determined the degree and rate of geomorphic change and thus change in aquatic habitat by analyzing spatially-rich data extracted from aerial photographs. Results show that a 10-km reach near Hatt Ranch narrowed 62% during a 44-year period. Analysis of temporally precise data measured at USGS gage 09328500 between 1949 and 1970, shows that the channel cross-section narrowed by 60% and incised its bed approximately 1.2 m. Rating relations since the 1980's provide corroborative evidence that channel narrowing and reduction in channel capacity continues; today, parts of the channel bed are on bedrock, thereby preventing further incision. We evaluated floodplain formation and channel narrowing processes by interpreting stratigraphy in floodplain trenches and dated these alluvial deposits using dendro-geomorphic techniques. We correlated changes in the flow regime with episodes of aggradation, thus determining the role of flood reduction in narrowing the channel. Stratigraphy observed in a 40-m long trench on Hatt Ranch demonstrates that the channel narrowed primarily by the deposition of sediment inset within the channel during the 1960's and 1970's. Dendrogeomorphic results verify that inset deposition is still occurring.

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Mar 30th, 1:40 PM Mar 30th, 2:00 PM

The Changing Geomorphic Template of Native Fish Habitat of the Lower San Rafael River, Utah

Eccles Conference Center

The physical template of the aquatic ecosystem of the lower San Rafael River changed drastically during the 20th century. Today, the lower San Rafael River still provides patches of complex habitat for roundtail cnub, Gila robusta robusta, flannel mouth sucker, Catostomus latipinnis, and bluehead sucker, Catostomus discobolus. Early 20th century photos show a wide, laterally unstable channel and a floodplain that is relatively low and was probably frequently inundated by overbank flows. 1938 aerial photographs also depict a complex, multiple thread, baseflow channel with numerous bars. During the subsequent 60 years, the channel simplified into a single-thread with low width-to-depth ratio. As a result of narrowing, the current channel contains few riffles and is only connected to the floodplain during rare, large floods. Aquatic habitat complexity is reduced. Three factors caused these changes in channel geomorphology: (1) reduced magnitude and duration of the spring snowmelt flood, (2) dense establishment of tamarisk (Tamarix spp) throughout the alluvial valley, and (3) continued supply of fine sediment from ephemeral tributaries. We determined the degree and rate of geomorphic change and thus change in aquatic habitat by analyzing spatially-rich data extracted from aerial photographs. Results show that a 10-km reach near Hatt Ranch narrowed 62% during a 44-year period. Analysis of temporally precise data measured at USGS gage 09328500 between 1949 and 1970, shows that the channel cross-section narrowed by 60% and incised its bed approximately 1.2 m. Rating relations since the 1980's provide corroborative evidence that channel narrowing and reduction in channel capacity continues; today, parts of the channel bed are on bedrock, thereby preventing further incision. We evaluated floodplain formation and channel narrowing processes by interpreting stratigraphy in floodplain trenches and dated these alluvial deposits using dendro-geomorphic techniques. We correlated changes in the flow regime with episodes of aggradation, thus determining the role of flood reduction in narrowing the channel. Stratigraphy observed in a 40-m long trench on Hatt Ranch demonstrates that the channel narrowed primarily by the deposition of sediment inset within the channel during the 1960's and 1970's. Dendrogeomorphic results verify that inset deposition is still occurring.

https://digitalcommons.usu.edu/runoff/2011/AllAbstracts/36