Resources to learn my local ecosystems?

I'm in Denver; I'd really like to deeply get to know my local environment, both the local urban environment and the wild environments around it. Walking around them and looking at things only gets one so far. What are the different kinds of plants? How do they interact with one another and with the climate? What is happening over time? How to tell if a creek is healthy or not? etc.

Are there groups or individuals in the area who host nature walks, etc. that would help one learn?

Find your chapter of the Sierra Club, they should be able to give you a schedule of their events.
Check to see if you have a mycology group for mushroom hunts.
Local Colleges and Universities usually have field trips for classes, you might be able to get a professor that would be willing to let you tag along.

Redhawk

I'm in Michigan and have found the USDA cooperative extension offices to be an invaluable source of information on local plants. I've picked up a lot of publications from them over the years on trees, native landscaping plants and wild edibles. They're the same folks that teach the master gardener program and can hook you up for info for pretty much everything that grows.