Two major development projects in Papua New Guinea, while still in the planning stage, are being approached with full awareness of their possible impact on the environment. A copper-mine high up in the catchment of a large river, the Fly, and a large hydroelectric dam on another sizeable river, the Purari, raise a number of questions. The extremely high rainfall in the area of the proposed copper-mine precludes the use of standard methods of tailings disposal. Thus if the tailings were stored behind a dam, it would frequently overflow; alternatively, they could be discharged directly into a stream. Both alternatives would have a negative environmental impact. Downstream dilution by tributaries would, however, be an important mitigating factor of tailing disposal. At present, frequent, naturally-occuring landslides in the upper catchment result in a very high input of sediments, with a disastrous impact on the aquatic life.