Joint Federal Interagency Sedimentation Conferenet and Federal Interagency Hydrologic Modeling Conference 2015

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Geomorphic Change in the Limitrophe Reach of the Colorado River in Response to the 2014 Delta Pulse Flow, United States and Mexico

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On March 23, 2014, a portion of the Colorado River was allowed to bypass Morelos Dam, the last dam on the river, and flow into the dry river channel of the Colorado River delta. This “pulse flow” was the result on an international agreement, Minute 319, that allowed Colorado River water to be used for environmental restoration, primarily to re-connect the river with the Sea of Cortez for riparian restoration purposes. The U.S. Geological Survey participated in hydrological and geomorphic monitoring of the pulse flow, with particular emphasis on the limitrophe, or border, reach of the river between Yuma, Arizona and San Luis Rio Colorado, Sonora. Here, we present results from the geomorphology and sediment transport components of the monitoring.
The geomorphology of the river corridor in the limitrophe can be divided into two major segments: an upstream segment in which the river channel is relatively confined within levees and a downstream segment in which the river channel is dry and less confined by levees. In the upstream portion, irrigation return flow results in a narrow, wetted channel with dense bank vegetation. In contrast, the downstream reach has an open sandy channel and less dense vegetation cover. Peak discharge during the pulse flow reached approximately 125 m3/s just downstream from Morelos Dam, but decreased to 70 m3/s at San Luis because of infiltration losses to the dry streambed. We used repeat measurements of channel cross-sections and bed sediment grain size to characterize changes in channel morphology at more than 15 sites within the Limitrophe. These measurements will be combined with pre- and post-pulse flow LiDAR data, and measurements of suspended sediment transport during the pulse flow, to document geomorphic change and sediment mass balance during the event.
Initial results and field observations suggest that most geomorphic change was confined to the pre-pulse active channel, which likely formed during floods in the late 1990s and is confined within higher geomorphic surfaces. Most of the repeat cross-sections show some geomorphic change, but some cross-sections were essentially stable during the event. Relatively little bank erosion was evident, particularly in upstream reaches where vegetation is most dense, but new sandbars formed in areas of flow expansion. There was little bed degradation during the event, even at sites immediately downstream from the dam, but there is evidence of streambed coarsening in these reaches. Farther downstream, areas of localized bed scour and deposition ranged from 10s of centimeters to more than a meter. Fluvial dunes that aggraded the bed 15 cm or more in many locations provide evidence of widespread bed load transport. The Colorado River delta pulse flow was a small hydrological event in the context of historic floods, thus largely isolating its geomorphic effect to localized erosion and deposition. Larger floods, such as those that last occurred in the mid-1980s and late-1990s, would be necessary to significantly rework higher geomorphic surfaces.

Author(s):

Erich Mueller    
Grand Canyon Monitoring and Research Center
U.S. Geological Survey

John Schmidt    
Grand Canyon Monitoring and Research Center
U.S. Geological Survey

David Topping    
Grand Canyon Monitoring and Research Center
U.S. Geological Survey

Paul Grams    
Grand Canyon Monitoring and Research Center
U.S. Geological Survey

 

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