During the past decade, airborne magnetic (aeromagnetic) surveys have been used increasingly to map within the sedimentary section of alluvial basins.  This new role contrasts with the traditional one that was limited to mapping deep crystalline basement at the basin floor.  Modern aeromagnetic surveys carry more sensitive instruments and are flown along lines that are closer to the ground and more narrowly spaced than in conventional aeromagnetic surveys.  These new high-resolution surveys allow detection of subtle magnetic contrasts in sediments and have increased ability to distinguish different igneous rock types within the top 500 m of the surface.  Although aeromagnetic data do not respond to the presence of water, they do contribute directly to understanding important geologic controls on ground-water systems.  In addition, aeromagnetic surveys cover wide areas cost effectively at a resolution commensurate with ground-water studies at basin scales.  Data from the surveys can be used to map the distribution of igneous rocks buried in the shallow subsurface, which are commonly found in alluvial basins.  Intrusive rocks may impede lateral flow, whereas volcanic layers can form a relatively impermeable base to alluvial aquifers above.  In addition, application of high-resolution aeromagnetic data to locate partially or wholly concealed faults within basin sediments has gained prominence in recent years.  Faults in sediments can compartmentalize aquifers and bound areas of land subsidence related to well pumping.  Aeromagnetic data commonly reveal many more of these faults within the basin than can be determined from surface evidence alone.  Examples of these uses of aeromagnetic methods come from studies of basins along the Rio Grande that are experiencing increased urban or agricultural demands on ground-water resources, from Albuquerque, New Mexico, to Alamosa, Colorado.