Alamos are cottonwood trees, so Los Alamos means The Cottonwoods. Los Alamos, New Mexico, is located in a high valley where aspen trees are common but cottonwood trees, which live at lower altitudes, are not. However there is a spring in the valley and a grove of cottonwoods are or were growing near it. The area got its name on maps from those trees because they were unusual and identified the area. (I know that alamos means poplar, not cottonwood, in modern Spanish. But the territory including what is now New Mexico was an isolated colony of Spain for several centuries, and an archaic dialect of Spanish was spoken there. There are other archaic words used in New Mexico -- laguna vs lago, prieta vs negro, others I have forgotten.