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- Floods 174
- Irrigation 21
- Hydrology 19
- Agriculture 14
- Watersheds 13
- Climate 11
- Precipitation (Meteorology) 11
- Rivers--Oregon 11
- Cities and towns 10
- Dams 10
- Groundwater 10
- Resource allocation 9
- Technology 9
- Water quality 8
- Finance 7
- Politics and government 7
- Canals 6
- Demography 6
- Drainage 6
- Droughts 6
