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- Explorers 242
- Discovery and exploration 231
- Northwest, Pacific 176
- Rivers 105
- Bodies of water 99
- River engineering 85
- United States. Congress--Records and correspondence 81
- Photography 57
- Riparian areas 35
- River surveys 33
- Rapids 32
- Harbors 23
- River channels 23
- Mountains 22
- Water-power 22
- Dams 21
- Canals 20
- West (U.S.) 18
- Aerial photographs 15
- Maps 14