The EPA is proposing water quality criteria in the San Francisco Bay and Delta of California ("Bay and Delta") to protect aquatic life and aquatic-dependent wildlife, including species listed as threatened and endangered under the federal Endangered Species Act, from the harmful effects of exposure to toxic levels of selenium. Selenium occurs naturally in California sediments, but can be concentrated and released into the environment through industrial and agricultural processes, and can negatively affect reproduction, growth and development in fish and waterfowl. Selenium is also known to bioaccumulate, such that a species' exposure to selenium is highly influenced by its feeding habits. In the Bay-Delta, selenium is efficiently bioaccumulated by the invasive filter-feeding clam Potamocorbula amurensis, commonly known as Corbula amurensis, causing particular risk to clam-eating fish and birds. This rule will improve water quality, protect aquatic life and wildlife, strengthen the natural ecosystem, and support outdoor recreation in the Bay and Delta region.