Many-Year Variations in River Delta Structures

Data on several river deltas are used to analyze the regularities in their dynamics in the context of variations of water and sediment runoff, sea level, and hydroengineering activities in delta areas. The basis for this analysis includes the results of many-year studies of river deltas in Russia and the world. The specific features of the evolution of the structure and morphometry of bayhead deltas, forming in bays, lagoons, and estuaries are shown in the case of the Alikazgan delta in the Terek mouth area and the deltas of two watercourses in the Mississippi mouth area. Data on many-year variations of the morphometric characteristics of modern protruding deltas in open coastal zones are systematized, and the factors that have an effect on these changes are analyzed. The types of delta formation processes and the types of deltas are considered with regard to the factors involved. The majority of modern river deltas are found to slow down their progradation into seas under the effect of anthropogenic runoff decline; moreover, some deltas have started retreating and degrading.