SURFACE AND SUBTERRANEAN DRAINAGE PIRACY, REORGANIZATION, AND KNICKPOINTS OF THE RÍO TANAMÁ AND RÍO CAMUY, PUERTO RICO

Recent mapping of caves in the Utuado Municipio has demonstrated that the two largest karst streams in Puerto Rico were originally a single river. An underground channel of the Río Camuy merged on-strike with the Río Tanamá at least 80 KaBP– as the latter exhumed a resistant basalt within carbonates to the north, incision stalled and the Camuy swivelled down-dip into a vertical series of at least five subsurface routes in the Lares Limestone. Subsequently, the loss of the Camuy (~50 cfs) reduced the Tanamá (presently 48 cfs) to a mean annual discharge less than required for maintenance of surface flow, and it has also been diverted underground. Other karst valleys in the humid tropics of Puerto Rico (where mean annual discharges are 2-3 cfs mile^-2) suggest this threshold to be about 60-80 cfs. Present knickpoints of the Tanamá largely correspond to the seven caves through which it flows.

Weak Tertiary clastics of the San Sebastián Formation have promoted growth of east-west strike valleys by sapping and collapse of overlying carbonates such as the Lares Limestone. The consequent northward migration of this impressive 150-250m scarp has also been a factor in the incremental beheading of holokarsts draining north to the Atlantic. Tributaries of larger neighboring rivers are extending their higher gradients along the strike-valleys at the southern karst contact to capture additional flow from the Tanamá and Camuy karst rivers, even in their upland non-carbonate catchments.