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.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "deepen":
   accelerate, agent provocateur, aggravate, amplify, annoy, augment,
   beef up, blow up, broaden, build up, burrow, complicate,
   concentrate, condense, consolidate, countersink, depress,
   deteriorate, dig, dig out, dive, double, dredge, drill, embitter,
   enhance, enlarge, exacerbate, exaggerate, exasperate, excavate,
   expand, extend, heat up, heighten, hop up, hot up, increase,
   intensify, irritate, jazz up, key up, lower, magnify, make acute,
   make complex, make worse, mine, mount, outspread, outstretch,
   provoke, ramify, redouble, reinforce, rise, rouse, scoop out,
   sharpen, sink, soup up, sour, spread, spread out, step up,
   strengthen, triple, tunnel, whet, widen, worsen