Effects of land use change on northeastern Puerto Rican rivers

Land use changes in northeastern Puerto Rico over the past 500 years increased the amount of water and sediment supplied to rivers causing them to adjust their size, shape and planform. Continued land development of the tropics makes it increasingly important to understand how and in what ways these rivers respond to changes in water and sediment supply.; Trends in land use were determined using a combination of historical data and aerial photographs. The last 500 years can be divided into four time periods based on land use: pre-European (pre-1500), Settlement (1500-1830), Agricultural (1830-1950), and Modern (1950-present). Water and sediment supply increased throughout the Settlement and Agricultural eras and decreased during the Modern era. During the Agricultural period, sediment yields increased more than water yields causing sediment supply to exceed transport capacity, which resulted in bed aggradation and storage of coarse sediment within the channels and fine sediment on the flood plains.; Channel cross-section geometry, cut-bank stratigraphy, and planform geometry of three rivers in northeastern Puerto Rico were measured using a combination of field surveys and analysis of aerial photographs. Surveys indicate that the quantity of sediment stored within modern channels increases in the downstream direction, and its elevation relative to the basal cobble layer of an older channel also increases in the downstream direction, suggesting that the Agricultural era sediment has been partially evacuated from the channel. Gage surveys confirm degradation in upstream reaches and aggradation in downstream reaches. Aerial photographs indicate that there is more bank erosion and channel avulsion in downstream reaches, further suggesting that these reaches have aggraded.; These rivers exhibit unusual trends in downstream hydraulic geometry. Channel depth and area both decrease in the downstream direction. Bankfull discharge also decreases in the downstream direction, whereas flood frequency increases. These trends are opposite to those normally observed in rivers, but are consistent with the existence of a large wave of sediment that has partially passed through the system.