GEOLOGIC AND DISCHARGE THRESHOLD CONTROLS ON KARST STREAMS IN PUERTO RICO

Flow direction in small rivers of the north central Puerto Rican karst is influenced dominantly by dip, strike, fault systems, basement topography, and discharge volume. After integrating on non-carbonate rocks, the Río Camuy sank underground in large caves mapped 3½ km on-strike, to merge eastward with the surface course of the Río Tanamá. Both followed a fault system 6 km NE as a single river before turning north on dip. After 100m of vertical incision a resistant basalt was exhumed prior to 80 Ka; the tributary Camuy abandoned the strike segment and swiveled north to a cave gradient directly down-dip to the Atlantic.

Data from other valleys in this karst suggest that a mean annual discharge of about 60-80 cfs is the threshold to sustain surface flow in the humid tropics, where areal flows are 2-3 cfs mile^-2. The loss of the Camuy (~50 cfs) reduced the Tanamá (now 48 cfs) below the limit of surface flow, and it was subsequently diverted underground. Abandoned stream terrace straths, and 2-3 levels in seven caves through which the Tanamá now flows, largely correspond to knickpoints in its present profile. The Río Camuy cave system records a vertical series of knickpoints in its six subsurface conduit levels.

Sapping and collapse of the Oligocene Lares Limestone overlying weak clastics of the San Sebastián Formation promotes a northward-migrating karst, with growth of east-west strike valleys at its southern boundary. Larger through-flowing rivers will complete holokarst development as they incrementally behead and further reduce catchments of the smaller streams.