The Collaborative International Dictionary of English v.0.48:

Deepen \Deep"en\, v. i.
   To become deeper; as, the water deepens at every cast of the
   lead; the plot deepens.
   [1913 Webster]

         His blood-red tresses deepening in the sun. --Byron.
   [1913 Webster]

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.
      [1913 Webster]

            It would . . . deepen the bed of the Tiber.
                                                  --Addison.
      [1913 Webster]

   2. To make darker or more intense; to darken; as, the event
      deepened the prevailing gloom.
      [1913 Webster]

            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.
      [1913 Webster]

   4. To make more grave or low in tone; as, to deepen the tones
      of an organ.
      [1913 Webster]

            Deepens the murmur of the falling floods. --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

deepen
    v 1: make more intense, stronger, or more marked; "The efforts
         were intensified", "Her rudeness intensified his dislike
         for her"; "Pot smokers claim it heightens their awareness";
         "This event only deepened my convictions" [syn:
         intensify, compound, heighten, deepen]
    2: become more intense; "The debate intensified"; "His dislike
       for raw fish only deepened in Japan" [syn: intensify,
       deepen]
    3: make deeper; "They deepened the lake so that bigger pleasure
       boats could use it"
    4: become deeper in tone; "His voice began to change when he was
       12 years old"; "Her voice deepened when she whispered the
       password" [syn: deepen, change]