As we can see on the question, in 1997 we had a level of 3827 and as the time passes in 2001 we just have a level of 3417.8 which means The surface elevation of Lake Powell is decreasing. Now to calculate the decay rate we just need to calculate how many years was passed (2001 - 1997 = 4 year) and the amount that has decreased (3827 - 3517.8 = 309.2), Now we can calculate the rate as follows:
So our final answer is:
The surface elevation of Lake Powell is decreasing at a rate of 77.3 feet per year.