There are a number of these puffballs in the riparian area above the dam, especially around the hummingbird tree (where the 2nd AZ Walnut is located). This one looks like a 'mature' (i.e., full of spores) puffball. If you step on it, a yellowish cloud will explode from within and there will eventually be more fungus among us. According to the mushroom expert, some puffball species can grow to be truly humongous fungus. Some species are edible, some are deadly. Just say 'no' to any chocolate-covered ones. Happy December!