Geomorphic response in the Limitrophe reach of the Colorado River to the 2014 delta pulse flow, United States and Mexico

Abstract:

In the spring of 2014, a pulse of high flow was released from Morelos Dam into the dry streambed of the Colorado River in its former delta. Although tiny in relation to typical delta floods of a century ago, this was the first flow to reach the sea since the 1998. Because much of the modern delta channel is now usually dry, this pulse flow was significant in its intention to benefit the riparian ecosystem. Mobilization of sediment in the typically dry sand bed and reworking of the channel bed and banks may provide opportunities for native plant establishment. Significant sediment transport also has the potential to affect infrastructure. Quantification of these geomorphic processes is thus critical in the planning of future pulse flows, should they occur. We used repeat measurements of channel geometry and bed sediment grain size, LiDAR, and measurements of suspended sediment transport to characterize geomorphic change and constrain the sediment mass balance during the pulse flow. We focused on the Limitrophe reach of the river, which represents the international border for 30 km downstream from Morelos Dam. The river corridor in the Limitrophe consists of an upstream segment in which nearby levees and irrigation return flow results in a narrow, wetted channel (active width ~25 m) with dense bank vegetation; and a downstream segment with an open sandy channel (active width ~85 m) less confined by levees or vegetated banks. Peak discharge reached 112 m³/s downstream from Morelos Dam, but decreased to 70 m³/s at the downstream end of the Limitrophe because of infiltration losses to the dry streambed. Measurable suspended-sand transport occurred throughout the Limitrophe, with export of sand out of the reach. Most geomorphic change was confined to the pre-pulse active channel, which is inset within higher geomorphic surfaces that formed during larger floods in the 1980s and 1990s. Sites immediately downstream from the dam did not degrade, despite active sediment transport, but the streambed coarsened. Farther downstream, localized bed scour and deposition ranged from 10s of centimeters to more than a meter, and fluvial dunes aggraded the bed in several locations. The pulse flow was small compared to historic floods, and larger floods would be necessary to significantly rework stable geomorphic surfaces or induce channel widening.