The echo will be stronger when originated from the bottom of the lake.
Step-by-step explanation
Echo is created when sound waves get reflected due to difference in the densities of the medium.
As we know that SONAR technique works on the detection of the echo strength to navigate, to determine the thickness of any object in water and also to measure the depth of the water bodies.
A region of warm water and cold water will create very negligible echo as there is no object which can resist the penetration of sound waves in this region.
But the sound waves will get reflected when hits the solid object or the bottom of the lake.
As the bottom of the lake is made up of solid, the sound waves will not be able to penetrate the bottom layer.
This will lead to reflection of sound waves creating echo of stronger energy.