Invasive species effects on water resources can be direct, as in the case of many aquatic nuisance species, or indirect, as in terrestrial species that change water tables, runoff dynamics, fire frequency, and other watershed attributes that in turn can alter water body condition. Beyond the dramatic estimate of $138 billion for yearly economic impacts and control costs, impacts to ecosystems and their beneficial services are estimated to be several times more than this total. Example watershed impacts include:
Cheatgrass now covers millions of acres of the Inter-mountain West, where it increases the occurrence of fires from 60-110 year frequency of small fires to a 3-5 year frequency over much larger areas. This increases fire-related impacts on water bodies from heavily burned watersheds; also cheatgrass reduces runoff in western watersheds due to its heavy use of early season moisture.
The Hemlock Woolly Adelgid is spreading throughout the Appalachians where in most areas it is 100% fatal to the Eastern Hemlock, a key species providing riparian forest cover and bank stability for headwaters streams. Deforested streams can then develop temperature and sediment problems and lose their coldwater aquatic communities.
Pepperweed invades wetland perimeters, riparian areas, salt marshes and other habitats in the West; it has interfered with regeneration of cottonwoods and willows, two key native species in western stream corridors.
Invasive species might cause indirect social/behavioral effects that could result in significant water quality impacts as well. For example, fear of non-native pests may prompt more pesticide and herbicide use and over-application, potentially increasing the amount of these chemicals entering water bodies through runoff.