Event Title

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

Presenter Information

Stephen Fortney

Location

Eccles Conference Center

Event Website

http://water.usu.edu/

Start Date

4-21-2010 11:40 AM

End Date

4-21-2010 12:00 PM

Description

The physical template of the aquatic ecosystem of the lower San Rafael River, a tributary to the Green River, changed dramatically during the 20th century. 1938 aerial photographs depict a channel comprised of multiple threads with numerous bars. The river has since been transformed into a single-thread channel with a low width-to-depth ratio. The drastic changes in the channel geometry have resulted in severely degraded habitat conditions. Despite these changes in habitat quality and quantity, roundtail chub, flannelmouth sucker, and bluehead sucker are still found in isolated patches of complex habitat. Three factors are primarily responsible for changes in the 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 of the San Rafael Swell. We determined the degree and rate of geomorphic change by analyzing spatially-rich data extracted from aerial photographs and temporally-rich data recorded at USGS gage 09328500. We evaluated and dated channel narrowing processes by interpreting stratigraphy in floodplain trenches and dated alluvial deposits using dendro-geomorphic techniques. Aerial photography analysis shows that a 10-km reach near Hatt Ranch cumulatively narrowed 62% during a span of 44 years. Between 1949 and 1970, the channel cross-section at USGS gage 09328500 narrowed by 60% and incised its bed approximately 1.2 m. Rating relations since the 1980's provide corroborative evidence that the channel continues to narrow; today, parts of the channel bed are on bedrock, thereby preventing further incision. Stratigraphy observed in a 40-m long trench demonstrates that the channel has narrowed by oblique and vertical accretion processes. These results will guide efforts to restore fish habitat and rehabilitate the San Rafael River by tamarisk eradication, in-stream flow augmentation, and reconnection of channel and floodplain habitats.

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Apr 21st, 11:40 AM Apr 21st, 12:00 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, a tributary to the Green River, changed dramatically during the 20th century. 1938 aerial photographs depict a channel comprised of multiple threads with numerous bars. The river has since been transformed into a single-thread channel with a low width-to-depth ratio. The drastic changes in the channel geometry have resulted in severely degraded habitat conditions. Despite these changes in habitat quality and quantity, roundtail chub, flannelmouth sucker, and bluehead sucker are still found in isolated patches of complex habitat. Three factors are primarily responsible for changes in the 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 of the San Rafael Swell. We determined the degree and rate of geomorphic change by analyzing spatially-rich data extracted from aerial photographs and temporally-rich data recorded at USGS gage 09328500. We evaluated and dated channel narrowing processes by interpreting stratigraphy in floodplain trenches and dated alluvial deposits using dendro-geomorphic techniques. Aerial photography analysis shows that a 10-km reach near Hatt Ranch cumulatively narrowed 62% during a span of 44 years. Between 1949 and 1970, the channel cross-section at USGS gage 09328500 narrowed by 60% and incised its bed approximately 1.2 m. Rating relations since the 1980's provide corroborative evidence that the channel continues to narrow; today, parts of the channel bed are on bedrock, thereby preventing further incision. Stratigraphy observed in a 40-m long trench demonstrates that the channel has narrowed by oblique and vertical accretion processes. These results will guide efforts to restore fish habitat and rehabilitate the San Rafael River by tamarisk eradication, in-stream flow augmentation, and reconnection of channel and floodplain habitats.

https://digitalcommons.usu.edu/runoff/2010/AllAbstracts/7