IMPEDED FLOOD DRAINAGE DUE TO RESTRICTED OUTLETS OF RIVER VALLEYS ALONG COASTAL PUERTO RICO: ITS IMPACT ON PRESERVATION OF PREHISTORIC ARCHAEOLOGICAL SITES

By the Late Holocene, prehistoric occupation within river valleys of the Greater Antilles was well established. Throughout time, this region has regularly endured intense tropical storms as well as widespread flooding, particularly in the lower coastal valleys. Alluvial stratigraphy in coastal valleys is complex due to the influence of coastal processes on fluvial regimes. To evaluate archaeological site preservation in these settings with precision, careful analysis of the nature of flooding and the depositional context of archaeological sites is necessary. Recent survey along coastal Puerto Rico recorded, mapped, and characterized the lower floodplain stratigraphy of 22 river valleys draining to the coast and their restriction by constrained spits or barrier formation across river outlets. Geologic, geomorphic, stratigraphic, hydrologic, and petrographic data indicate that annual floodplain deposition is complex because it was produced by the interplay between fluvial and coastal processes behind coastal drainage restrictions. Depending on the degree of outlet restriction, commonly, a lagoon formed adjacent to the coast. During periods of high discharge, increased discharge backed up from coastal lagoons causing rise in upstream channel flow until it spilled over their banks. Adjacent floodplains consist mainly of thick overbank deposits of fine textured sediments. This suggests that prehistoric sites buried within floodplains and terraces have high potential for preservation. Despite the seasonal impact of tropical storms, hurricanes, and severe flooding, even landscapes within these settings can provide high levels of archaeological preservation depending on the geomorphic factors involved.