Subject
United States Discovery and exploration
7
Colorado River (Colo.-Mexico)
6
Grand Canyon (Ariz.)
6
Colorado River (Colo.-Mexico) Discovery and exploration
4
Explorers
4
Grand Canyon (Ariz.) Description and travel
3
Grand Canyon (Ariz.) Discovery and exploration
3
Colorado River (Colo.-Mexico) Description and travel
2
Conservationists
2
Geologists
2
Arid regions
1
Discoveries in geography American
1
Long distance swimming
1
Philosophy
1
Southwest, New Description and travel
1
Southwest, New Discovery and exploration
1
Swimmers
1
Travel
1
West (U.S.) Description and travel
1
West (U.S.) Discovery and exploration
1
West (U.S.) Environmental conditions
1