Potential hydrologic effects of a drainage system in McMillan delta and water impoundment in Brantley Reservoir, Eddy County, New Mexico

Construction of a proposed drainage system could result in a moderate flow increase in the Pecos River downstream from the McMillan delta. The potential effect of a new line channel of the Pecos River in McMillan delta in southeastern New Mexico would be an increase of less than 11,000 acre-ft/year. This increase includes overflow of 300 acre-ft from the present Pecos River channel, seepage losses of 3,600 acre-ft from the river bed and tributary inflow of 7,100 acre-ft. The potential effects of drains at the north end of the study area would be additional water of about 6,100 acre-ft within the first few years. In order to drain this much water, the drains would have to be dredged to a lower depth 6 to 8 mi to the south. Impoundment in Brantley Reservoir will cause increases in groundwater storage. The quantity of increased storage will depend on average reservoir pool levels. Major Johnson Springs probably will cease to flow at the conservation-pool level, and southward groundwater leakage from the Major Johnson Springs aquifer could increase. Large quantities of water may move in and out of storage in the Major Johnson Springs aquifer as the Brantley Reservoir pool changes between minimum pool and conservation pool levels. A ground--and surface-water monitoring network is needed to determine changes in groundwater storage caused by Brantley Reservoir. Water levels in selected wells need to be measured periodically during operation of the reservoir. Additional streamflow-gaging stations need to be established and surface-water samples analyzed to determine changes caused by a drainage system and Brantley Reservoir. (USGS)