The Ever-Changing Delta
The Ever-Changing Delta
After his visit to Egypt in the year 500 B.C.E., Herodotus compared the triangular shape of the lowland region, where the Nile and sea meet, to the Greek letter Δ, thereby introducing the term delta to the geographic literature. In Chapter 1, we defined a delta as “a discrete shoreline protuberance formed where a river enters an ocean or lake … a broadly lobate shape in plain view narrowing in the direction of the feeding river, and a significant proportion of the deposit … derived from the river.” Coastal deltas are geologic structures that are also subcomponents of an estuary, which is commonly defined as a semienclosed body of water, situated at the interface between the land and ocean, where seawater is measurably diluted by the inflow of fresh water. James Syvitski, a world-renowned expert on deltas, describes how a delta’s area can be defined as “1) the seaward prograding [building outward] land area that has accumulated since 6,000 years, when global sea level stabilized a few meters of present level, 2) the seaward area of a river valley after the main stem of a river splits into distributary channels, 3) the area of a river valley underlain by Holocene marine sediment, 4) accumulated river sedi¬ment that has variably been subjected to fluvial, wave, and tidal influences, 5) the area drained by river distributary channels that are under the influence of tide, or 6) any combination of these definitions.” These delta-front estuaries, hereafter referred to as deltas, are dynamic ecosystems that have some of the highest biotic diversity and production in the world. Consequently, an estimated 25% of the world’s population lives in environments that are coastal deltas and their associated estuaries/ wetlands. Deltas provide not only a direct resource for commercially important estuarine species of fishes and shellfish but also shelter and food resources for commercially important shelf species that spend some of their life stages in estuarine marshes. For example, high fish and shellfish production in the northern Gulf of Mexico is strongly linked with discharge from the Mississippi and Atchafalaya river delta complexes and their associated estuarine wetlands.
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