Recently the SCOTUS made rulings on the clean water act that have strongly restricted the ability of the EPA to prosecute companies that pollute the nations waters, as reported in the NY Times. The reason is that the law states that it is limited to "the discharge of pollutants into the navigable waters". The course has recently stated that this means that ephemeral streams (those that occasionally go dry) or lakes not connected to other bodies of water may not be "navigable waters". There is even the idea that watersheds that are entirely within a single state may also be exempt from the federal law. Adding to that, the EPA is limited in its ability to prosecute polluters due to difficulties of proving jurisdiction. The article also reports that in the last 4-5 years, the number of violations has been on the rise, but that the EPA may be unable to prosecute as many as half of the nations largest polluters.
Opinion: this ruling is absolutely dreadful and appalling. It gives a free pass to many to dump toxic substances into water, which is certainly cheaper than paying for proper disposal. But in the end, it will lead to a weak and polluted ecosystem that will eventually raise the level of cancer in ourselves. Despite whether waterways are "navigable", water has an incredible ability to scatter pollution over a wide area.