Rates of sediment supply and sea-level rise in a large coastal lagoon

Laguna Madre, Texas, is 3-7 km wide and more than 190 km long, making it on e of the longest lagoons in the world. The lagoon encompasses diverse geolo gic and climatic regions and it is an efficient sediment trap that accumula tes elastic sediments from upland, interior, and oceanic sources. The semi- arid climate and frequent tropical cyclones historically have been responsi ble for the greatest volume of sediment influx. On an average annual basis, eolian transport, tidal exchange, storm washover, mainland runoff, interio r shore erosion, and authigenic mineral production introduce approximately one million m(3) of sediments into the lagoon. Analyses of these sediment t ransport mechanisms and associated line sources and point sources of sedime nt provide a basis for: (1) estimating the long-term average annual sedimen t supply to a large lagoon; (2) calculating the average net sedimentation r ate; (3) comparing introduced sediment volumes and associated aggradation r ates with observed relative sea-level change; and (4) predicting future con ditions of the lagoon. This comparison indicates that the historical averag e annual accumulation rate in Laguna Madre (<1 mm/yr) is substantially less than the historical rate of relative sea-level rise (similar to 4 mm/yr). Lagoon submergence coupled with erosion of the western shore indicates that Laguna Madre is being submerged slowly and migrating westward rather than filling, as some have suggested. (C) 2000 Elsevier Science B.V. All rights reserved.