Estimate how much volume is in a circular lake that is 1km wide and 10 meters deep.

How deep at the edges?

First determine the area of the lake, then multiply that by it's average depth. Area of a circle is \pi D^2/4. Since D =10^3 m, and you can estimate that \pi/4 is "about" 3/4, so the area is around 0.75 x 10^6 m^2. Multiply that by the estimated depth of 10 m, and what do you get?