Utilizing Hydrophones To Detect Streambed Mobilization In The Wild and Scenic Reach of The Rio Chama

The segment of the Rio Chama between El Vado Dam and Abiquiu Dam in New Mexico was designated as a Wild and Scenic River in 1988. The Rio Chama is a high-sediment system with a significant amount of fine sand, silt, and clay sediments deposited by inflows from arroyos and bank erosion onto gravel-cobble (i.e., coarse) bed material. The fine sediment fills the interstitial spaces within the gravel streambed and adversely impacts the suitability of spawning habitat of the brown trout, a target management species in the Wild and Scenic segment.

Bureau of Reclamation periodically releases high flow pulses from El Vado Dam for environmental enhancement purposes, including clearing fine sediment from the streambed to maintain habitat for brown trout and their macroinvertebrate food base. Fine sediments are mobilized, along with coarser sediment, during incipient motion which is said to begin when a flow has enough energy to pick up the median grain size in the streambed. Mobilization of the streambed is necessary to remove fine sediments on top of the gravels as well as the fines in interstitial spaces to promote dissolved oxygen flow through the bed.

The Rio Chama receives a combination of native water and inter-basin transfer water from the San Juan-Chama Project. Because of climate change, the Rio Chama is projected to see native water flows decrease by one-third and inter-basin transfer flows by one-quarter over the next century. With less water available in the future for high flow pulses, it is necessary to better understand what flow rates are needed to trigger incipient motion. This knowledge would assist water managers in planning the most effective high flow pulses.

Previous research has shown hydrophones to be a viable passive method to monitor sediment transport and incipient motion. Because incipient motion is a threshold event, the hydrophone recordings are able to capture the transition from the relative quiet of a stream during normal flow conditions versus a noisier stream during incipient motion, when coarse sediments are colliding with each other. Correlating these transitions to changes in flow rate would indicate what flow rates initiate incipient motion. This project seeks to expand the applicability of underwater acoustic sensors, or hydrophones, to the Wild and Scenic Rio Chama to test its capabilities there and potentially develop a long-term sediment mobilization monitoring methodology. The project is a collaboration between the Bureau of Reclamation and the United States Geological Survey.