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- Lectures and lecturing 382
- Rivers--Oregon 36
- Dams 31
- Oregon--History 21
- Sagan, Carl, 1934-1996 21
- Thomas Condon 21
- University of Oregon 21
- United States. National Aeronautics and Space Administration 20
- Water--Pollution 20
- Wildlife refuges 20
- Astronomy 19
- Conservation of natural resources 19
- Birds--Conservation 18
- Oregon Wildlife Federation 17
- Salmon 16
- Fishes--Conservation 13
- Izaak Walton League of America 10
- Hunting 8
- Bears 5
- Ducks 5