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- Canals 277
- Irrigation 74
- Politics and government 60
- Dams 54
- Agriculture 40
- Building 40
- Rivers 34
- Finance 26
- Hydrology 25
- Water diversion 25
- International regimes 20
- Water-power 20
- Bodies of water 19
- River engineering 19
- Bureaucracy 18
- Economics 18
- Rivers--Oregon 18
- Armed Forces 17
- Northwest, Pacific 17
- United Nations 17
