It probably differs from area to area, but look up wildland volunteer groups. At least here we have state-level ones, county and city ones, private orgs, ones devoted to specific watersheds and ranger districts, and ones that specialise in different kinds of work/seasons. With mine it's protecting a specific ranger district so you can use the web portal to sign up for patrolling individual hikes in it or for group days where we might be picking invasive weeds or rebuilding a bridge or doing a guided hike with a geologist member. Most of us are some kind of natural scientist or at least devoted naturalist so it's super fun to experience nature with passionate experts.