GROUNDWATER GRADIENT REVERSAL DURING FLOOD EVENTS ALONG A CRITICAL REACH OF THE RIO GRANDE

By using a highly instrumented 50-mile reach of the Rio Grande, we are able to see how groundwater responds to changes in the river stage. This critical reach on the Rio Grande has 154 wells, 19 stream gages, and 70 pressure transducers that record hourly water levels, providing a detailed look at flood events. This high-resolution data shows a vertical gradient between the phreatic aquifer and the semi-confined aquifer that reverses during flood events. Gradient changes during flooding confirm the presence of a low permeability layer, at a depth varying between 20 and 50 feet, identified during well installation. A preliminary surface water/groundwater interaction model depicts the low conductivity stratum as a continuous layer. The detailed flood data is being used in a version of the numerical model to better explain the relationship between this semi-confining layer and the reversible vertical gradient.