I think over the last few years trash left purposefully (with the exception of fireworks) has generally decreased. One of the most common trash heaps we find are fire rings with melted glass, aluminum, and plastic left in them. Yes, all three items distort in a fire, but that doesn't mean they are gone. To some folks, out of sight, out of mind, I suppose. Same with fireworks that litter the ground, which is by far the item we pick up the most. On my last trip in July, my team and I picked up hundreds (literally) of 1/4 to 1-inch pieces (as well as some of the large pieces) of fireworks strewn about a beach in Wilson Creek on the San Juan. To me, Lake Powell visitors who put on the firework displays for all to enjoy (or disrupt a beautiful and quiet starry night) are just as much litterbugs as someone who finishes a beer and throws the soda can over the side of the boat, or leaves a heap of melted trash in a fire ring as they drive away from a beach, or chucks cigarettes and trash out of their car window on the highway. Littering is littering, and if you are one of the people putting on displays, you are part of the problem. Hopefully some Trash Trackers will clean up your mess.
Eric