The most significant environmental impacts in Jefferson caused by humans are habitat destruction, pollution, and water use.
While most of Jefferson is rural and mountainous, some areas are developed. Cities like Chico, Eureka, and Medford all require large areas of land, and destroy natural habitats in and around their locations. The ecological footprints of these cities combined with the rural settlements cause further environmental impacts to the area in the forms of pollution and waste. Economic prosperity of the region depends largely on resource extraction, particularly logging. This, of course, leads to losses in habitat as well.
Water use also has a significant impact on the environment in Jefferson. The damming of major rivers has led to a decrease in salmon population, and the transportation of water to Los Angeles via aqueducts has left parts of Jefferson with less water than has been available historically.