Archaeological excavations at the coast of A'asu, in Tutuila Island of American Samoa, exposed a depositional sequence spanning the past circa 700 years. With the period represented, sedimentation rates exceeded 10.15 cm per century in the valley floor and 16.34 cm per century along the valley margin. The occupational history may correlate with changes in climate, sea level, and coastal geomorphology. Although the evidence accords with the expected responses to the Little Climatic Optimum (circa 1050 to 690 B.P.) and Little Ice Age (circa 575 to 150 B.P.), the most plausible explanation for the A'asu case is that environmental change accompanied expansion of upland land use. Based on evidence here and elsewhere in Tutuila, it is proposed that the establishment of fortifications, monuments and permanent settlements in the uplands was part of a broader pattern of land-use expansion beginning in the fourteenth century A.D.