Seepage investigation of the Rio Grande from below Leasburg Dam, Leasburg, New Mexico, to above El Paso, Texas, 2018

Seepage investigations were conducted periodically by the U.S. Geological Survey (USGS) from 1988 to 1998 and from 2006 to 2015 along a 64-mile reach of the Rio Grande as part of the Mesilla Basin monitoring program. Past studies were conducted during no-flow or low-flow periods. In 2018, a seepage investigation was conducted during April 3–4 along a 62.4-mile study reach, from below Leasburg Dam, Leasburg, New Mexico, to above El Paso, Texas, during a period of high flows due to dam releases of water for irrigation purposes. During this investigation, there was measurable streamflow at 31 of the 41 measurement locations: 22 river sites, 8 inflow sites, and 1 outflow site. Results of the 2018 high-flow seepage investigation are presented in this report.

Net seepage gain or loss was computed for each subreach (the interval between two adjacent measurement locations along the river) by subtracting the streamflow measured at the upstream location from the streamflow measured at the closest downstream location and then subtracting any inflow to the river within the subreach. An estimated gain or loss was determined to be meaningful if it exceeded the cumulative measurement uncertainty associated with the net seepage computation. During this investigation, streamflow on the main stem of the Rio Grande ranged from 577 to 1,000 cubic feet per second (ft³/s). Nine subreaches were found to have meaningful net seepage gain or loss, four gaining subreaches and five losing subreaches. Because of high cumulative uncertainty (plus or minus 111.3 ft³/s) relative to the calculated cumulative loss (−57.7 ft³/s) over the entire study reach, no meaningful gain or loss was determined in this study. Like all of the previous USGS seepage studies on this reach of the Rio Grande, this study reported a net seepage loss, and the magnitude of that loss was within the range of historical values.