地球日提醒我们要保护图中这种干净的河流

出处: "Silver Cascade--White Mountains." Currier & Ives, between 1840 and 1880. Prints and Photographs Division (in Documentary Chronology of Selected Events in the Development of the American Conservation Movement, 1847-1920, The Evolution of the Conservation Movement, 1850-1920), Library of Congress.