One of the greatest sources of chemical pollution that Washington faces is runoff from large scale agriculture. In residential areas, herbicides, insecticides, and fertilizer from yards and gardens can also cause major problems in local waters.
Herbicides can cause plant death of both terrestrial and aquatic plants. Other than depleting the ecosystem, many cold water fish species (salmon, trout, and their eggs) rely on the shade of adjacent plants to keep cool.
Insecticides cause two main problems. Firstly, they can directly cause the death of many crucial insect species. They are designed to harm terrestrial insects but aquatic insects are affected as well. Additionally, many aquatic species (fish, birds, and other insects) are insectivores, meaning they only eat insects. If their food supply is depleted, their abundance will decline proportionally.
Fertilizers are typically enriched with many types of nutrients like nitrogen, phosphorous, and ammonia. While these do help many plants in your garden that require the additional nutrients, once they enter a natural ecosystem, they can cause overgrowth problems. These additional nutrients allow for plants like algae and moss to overgrow and create algal blooms (large areas covered with algae). When this happens, the extra plant material is photosynthesizing additional oxygen and reducing the amount available in the water for other species like fish.