The environmental impact of population growth in the US is likely to be different from the impact of growth in developing countries. Population growth in the US is more likely to have a global impact while population growth in less developed countries will have more of a local impact.
In less developed countries, population growth tends to hurt the local environment. Inadequate or nonexistent sewer systems mean that larger populations pollute the water more. The need for more food means that areas of forest are cut down and marginal areas like hill sides are farmed. This can lead to problems like landslides and the silting-up of streams. These are local problems that do not affect people outside the local area very much.
In the US, population growth tends to hurt the global environment, often in less tangible ways. This is because Americans are so rich and consume so much. More Americans means more cars and more air conditioners using energy. More Americans means more demand for goods made in China, which leads to more coal being burned in Chinese power plants. Since Americans consume so much, they use a lot of energy. This causes more greenhouse gases to be emitted, increasing the likelihood of serious global warming.
Thus, population growth in a less developed country has more of a local impact while growth in America (or other rich countries) is more of a long-term danger to the environment of the entire world.