Since there is no word "shallowen", is there a good antonym of "deepen"? The meaning of "deepen" that I am referring to here is "to cause to become deeper." It doesn't necessarily refer to digging a hole. For example, a temporary deformation in a soft surface:
Putting the anvil on the mattress deepens the depression in it, but removing the anvil ______ it.
I would also be happy with an antonym of the intransitive version "to become deeper."