To find the elevation of base camp, we start with Helene's initial elevation of 33 feet above sea level and subtract the 46 feet she descends.
Calculation: \[ 33 \text{ feet} - 46 \text{ feet} = -13 \text{ feet} \]
This means the elevation of base camp is 13 feet below sea level.
So, the correct description of the elevation of base camp is: 13 feet below sea level.