三角洲的建造是流域系统内侵蚀,搬运和沉积过程的有机部分。对以多沙特性著称的黄河来说,这个地质作用尤其显著,每年几乎有五分之三的泥沙沉积在河口区,致使三角洲迅速地向渤海大陆架推进。

Due to the frequent shifting of its distributary mouth and displacempnt of its apexthe Yellow Kiver delta has a complex structure.1. The environment factors which influence the delta formation are:1) Sediment yield of the river basin2) Fluctuation of the sea-level3) Marine Dynamics4) Sea-bottom features5) Frequent shifting of the lower channel2. The geomorphological structure of the delta and their genesis1) The delta plain declines from southwest towards northeast with an averagegradint of 0.1-0.15 ‰　. It consists of distributary channels and bordering levees, com-posed of fine sands and silts with intervening depressions composed of fine clay and silts.The plain is formed as a result of frequent shiftings of the distributary channels. Since the latter are intensively aggraded and often breached at certain points, the amount of sediment deposited on the delta plain occupies about 20% of the oncoming sediment discharge of the Yellow Kiver.2) Undersea delta: It is the extension of the delta plain under the sea and spreads out towards the shallow sea in the from of a fan. Its average gradient is 0.8%0. It is formed in the flat and shallow off shore enrironment where tide and wave are both very weak. Here the water flow from the river is dispersed with a consequent decrease of flow velocity thus reducing its sediment carrying power both this fact and the floc-culation of fine sediments by saline water, lead to large amount of sedimentation in this area which attains 40% of the total oncoming load from the Yellow River.3) The river mouth bars: Checked by the tide, the sediment poured into the river mouth area is partly laid down to form a series of sand bars. Some of them stretch across the channel mouth as transverse mouth bars; some of them stretch along the channel as mid-channel bars; some of them lie beyond the channel mouth as offshore bars.3. The evolution model of the deltaThe evolution model of the Yellow River delta can be outlined as follow: two main stages of evolution have been recognized, the formation of the first major subdelta during 1889?934 with its apex at Ninghai and the formation of the second major subdelta during the period of 1934?980 with its apex of Yuwa. With each of thesetwo major subdelta, there are five tongue like minor subdelta. The evolution cycle for each major subdelta is about 50 years, while that for each minor subdelta about 7 years. These subdeltas of different scales and different ages are all overlapping one anather. This is revealed in vertical sections of the delta alluviums and deposits. Under the alluvium at the front of each subdelta lobe, a buried lens of marine deposits can often be found.4. The effect of the delta evolution on the upstream reaches of the river With the apex of the delta being relatively fixed. The surface of the delta has been raised with corresponding elevation of the river bed upstream from the delta area. This may be called the headward aggradation along the river, But whenever a shifting of the channel occurred or when there was abundant water in some the years, both downstream and heaklward degradation of short period would be found from the apex of the delta of that time. Successive rising of the river bed near Yuwa and their correspendiug headward aggradation were very remarkable during the period from 1969?973, besides, headward aggradation has also a close relation with the rising of the average sea-level. The coefficient of correlation is r=0.95 during 1970?1973. Judging from the adjustment of the long profile of the river and the distribution of D₅₀ grain-size along the river. We find that the headward aggradation has almost reached near Lao Kou, about 262 Km upstream from the mouth.