Effects of land-use change on channel morphology in northeastern Puerto Rico

Between 1830 and 1950 much of northeastern Puerto Rico was cleared for agri culture. Runoff increased by similar to 50% and sediment supply to the rive r channels increased by more than an order of magnitude. Much of the land c learance extended to steep valley slopes, resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels. A shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supp ly has decreased, allowing the rivers to begin removing coarse sediment sto red within their channels. The size, abundance, and stratigraphic elevation of in-channel gravel bar deposits increases, channel depth decreases, and the frequency of overbank flooding in creases downstream along these channe ls. This is presumed to be a transient state and continued transport will l ead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment. A downstream decrease in chann el size is contrary to the expected geometry of self-adjusted channels, but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period. Reverse (downstream decreasing) chann el morphology is not often cited in the literature, although consistent obs ervations are available from areas with similar land-use history. Identific ation of reverse channel morphology along individual watercourses may be ob scured in multiwatershed compilations in which other factors produce a cons istent, but scattered downstream trend. Identification of reverse channel m orphology along individual streams in areas with similar land-use history w ould be useful for identifying channel disequilibrium and anticipating futu re channel adjustments.