All the answers below are ...

All the answers below are possibilities.  If the bedrock contains a limestone horizon which can be prone to the formation of sinkholes, one or more sinkhole collapses over the past decade could be diverting surface water from the monsoon into a fractured aquifer or cavern system.  Examination of the ground surface in the vicinity of the major streams for evidence of ancient sinkholes may be possible but this effort will unfortunately be impeded by the forest cover you describe.  This situation, if true, would be very difficult to fix.  "Civilization exists by geologic consent, subject to change without notice".  [Will Durant] Good luck. 

J. Gusek