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- Explorers 431
- Discovery and exploration 380
- Northwest, Pacific 309
- River engineering 288
- Rivers 259
- Bodies of water 212
- West (U.S.) 194
- Government publications 186
- Photography 160
- United States. Congress--Records and correspondence 138
- United States--Appropriations and expenditures 115
- Canada, Western 107
- River surveys 71
- Rapids 55
- Railroad engineering 48
- Transportation 42
- Inland navigation 41
- Canals--Design and construction--Costs 38
- Dams--Design and construction 33
- Tourism 28