As you travel higher in elevation from sea level the air gets thinner and the atmospheric pressure is reduced. This means as you make your drive up to Clear Lake you will in essence be surfacing from your first dive of the day. As a result you will have some residual nitrogen (RNT) accumulation. This is why at 3000' you should treat yourself as a "C" diver upon arrival (according to NAUI tables). Then calculate your first wet dive based on a surface interval after arrival. Most dive computers today have an adjustment for altitude but cannot account for the residual nitrogen accumulated during your trip up. So unless your schedule can allow for a lengthy initial surface interval it is best to use Tables to calculate dive times.
Some depth gauges are calibrated for sea level and salt water. This means if you have one of these gauges your depth readings should be compensated accordingly. Digital dive computers with altitude adjustments automatically compensate.
One last thing to consider, your dive tables are based on salt water and sea level. We will be diving at 3000' of elevation in fresh water. This will require an equivalancy adjustment for table depths. This also effects safety stop depth and ascent rates.
From NAUI Manual