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- Blackfoot River 2 results
- Bolivia 2 results
- Boston 2 results
- Brazil 8 results
- British Columbia 2 results
- Cahaba River Region 2 results
- Cahaba River Region (Ala.) 2 results
- California 44 results
- California, Northern 2 results
- Cambridge 6 results
- Cambridge (Mass.) 4 results
- Cameroon 2 results
- Canada 36 results
- Caribbean Area 4 results
- Caribbean Sea 2 results
- Carlsbad Region 6 results
- Carmel River Region 2 results
- Cebu City 2 results
- Centralia 2 results
- Centralia (Pa.) 2 results
- Chesapeake Bay Region (Md. and Va.) 4 results
- Chesapeake Bay Watershed 2 results
- Chicago 2 results
- Chicago River 2 results
- Chile 6 results
- China 78 results
- Coeur d'Alene River 2 results
- Coeur d'Alene River (Idaho) 2 results
- Cold regions 6 results
- Colombia 2 results
- Colorado 18 results
- Colorado River (Colo.-Mexico) 4 results
- Connecticut 2 results
- Croatia 2 results
- Cuba 4 results
- Côte d'Ivoire 2 results
- Dearborn 4 results
- Deep River (Ont.) 2 results
- Delaware 2 results
- Delta Region 2 results
- Delta Region (Calif.) 2 results
- Denmark 2 results
- Developed countries 6 results
- Developing countries 70 results
- Ducktown 2 results
- East Asia 4 results
- East Liverpool 2 results
- Ebro River Watershed 2 results
- Ecuador --Oriente 2 results
- Egypt 4 results more…