Often there is already a depression you can use as is. Or deepen, and use the excavation for a dike. If not, maybe there is a box canyon you can close the end

Often there is already a depression you can use as is. Or deepen, and use the excavation for a dike. If not, maybe there is a box canyon you can close the end of with a dam. In places, they have flattened a convex hilltop and used the material to make a dike. Usually you don't need anywhere near as much storage volume as a typical hydroelectric dam reservoir holds.