The Collaborative International Dictionary of English v.0.48:

Deepen \Deep"en\, v. t. [imp. & p. p. Deepened; p. pr. & vb.
   n. Deepening.]
   1. To make deep or deeper; to increase the depth of; to sink
      lower; as, to deepen a well or a channel.
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            It would . . . deepen the bed of the Tiber.
                                                  --Addison.
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   2. To make darker or more intense; to darken; as, the event
      deepened the prevailing gloom.
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            You must deepen your colors.          --Peacham.
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   3. To make more poignant or affecting; to increase in degree;
      as, to deepen grief or sorrow.
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   4. To make more grave or low in tone; as, to deepen the tones
      of an organ.
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            Deepens the murmur of the falling floods. --Pope.
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