Large national parks, established in the early 1900s, have provided a continuing source of revenue; tourism has grown steadily. During World War I the price of silver soared again and the economy prospered. The stock-market crash of 1929 and the droughts of 1935 and 1937 brought hardships, but the economy recovered again during World War II, when the state’s foods, minerals, and metal products were important to the war effort.

In the mid-1960s Colorado experienced a large influx of new residents and rapid urban growth and development, especially along a strip (c.150 mi/240 km long) centered on Denver and stretching from Fort Collins and Greeley in the north to Pueblo in the south. This growth, combined with the area’s high altitude, caused pollution problems, most notably smog. The discovery and exploitation of oil created a boom in the 1970s, which collapsed in the early 1980s. Diversifying industry, swelling in-migration and accompanying construction, and tourism and recreation have since enabled Colorado to rebound, and between 1990 and 2000 it had the third largest percentage of growth of any state in the union.