I could be all wet on this (sorry), but I think there is a seperate drain valve used to drain the tank completely. The spout valve is several inches above the floor of the tank.
Mud in water tanks is one of things one has to deal with. I remember telling everyone that stopped at Cresco for water to pull the spout down over the closed hatch, pull the rope and let the mud out of the bottom of the tank, before filling up. If you get a heavy rain above Cresco, the tank will mud up. The mud would also foul the intake box above the highway. I don't recall that kind trouble at Los Pinos. The rest of the tanks were spring fed which insulated them from mud issues.