The service vehicles seen are not drudging, if you notice there is a chug line across the ground. The chug line is a catch for any large debris like trees, before it goes into the dam's flow grates. The park cleans all of that stuff out a couple times a year, or whenever it needs it. Inside the dam I saw the gate pistons that operate the grates. There are 12 sets in total, each set is about a foot in circumference, and 6 feet high. The least to say is they were pretty sweet to see.