While most discussions about what transit can do for the environment almost always revolve around climate change, transit is part of a larger set of tools that are critical to save the Puget Sound. Frontline has a great documentary that looks at the environmental challenges that both the Chesapeake Bay and the Puget Sound are facing.
For the Puget Sound one of the most significant threats is surface runoff caused by impervious surfaces. Limiting sprawl and creating dense, and thus more transit dependent communities is one part of the solution. Reducing runoff through Low-Impact Development (LID) is another. I’m no expert and I know just enough to be dangerous so I’ll leave it there but certainly check out the documentary. Its very informative, both about what causes polluted waters as well as government regulations, or lake there of.
Also, if you’re not familiar with the The Puget Sound Partnership or The Cascade Land Conservancy and their plans, the Puget Sound Action Agenda (better website here) and the Cascade Agenda respectively take a quick look.