Yes, the tunnel pulls from deep in lake dillon. Deep water = cold water, but it still warms up in the summer (like everything else). Check the link below and scroll down til you find the temp vs. depth vs. month plot and that pretty much explains it. Cold water will stay on the bottom of the lake because cold water is more dense than warm water, but the heat of the summer will warm the entire lake up, even the bottom. Depends on the year, but the difference in temp of roberts tunnel water could be 4-5F in may vs. aug (~35F in may... ~40F in aug).