Changing Middle Rio Grande Channel Conditions: Montano Road Bridge to Isleta Diversion Dam

The Montano Reach of the Middle Rio Grande spans about 19 miles from the Montano Bridge Crossing in Albuquerque, New Mexico, downstream to Isleta Diversion Dam. The analysis of spatial and temporal trends in channel geometry and morphology are reported including dynamic hydrologic and hydraulic changes over the past century. The mean annual discharge has decreased since the onset of a regional drought in about 2000 and the suspended sediment concentration decreased after Cochiti Dam began impounding water in 1973. Upstream dam construction caused channel incision ranging from about 2.5 ft near Montano Bridge (about 50 miles downstream of Cochiti Dam) and decreasing to zero upstream of Isleta Diversion Dam that acts as a grade control. Bed lowering in the upper 14.7 miles of this reach led to a slight increase in flow depth, velocity, and decrease in bed slope. The bed elevation has remained about the same in the 4.3 miles upstream of Isleta Diversion Dam. Median bed grain size has increased from 1972 to 2012. Sinuosity has decreased from about 1.08 to about 1.04. A GIS analysis of maps and aerial photographs dating as far back as 1918 shows geomorphic changes where the channel width has decreased from about 1,470 to about 380 ft wide; it is now about 1/4th of the historical width. Channel planform has changed from a braided, highly mobile, sand bed condition to a narrow, single thread channel bed. To further understand the channel evolution, a geomorphic conceptual model based was used. Geomorphic changes in the Montano Reach demonstrate how a river system responds to complex and interrelated factors such as dam construction, urbanization, and alterations to the flow and sediment regime. Understanding and documenting these changes informs management decisions and river restoration and flood protection projects on the Middle Rio Grande.