Abstract:

The Boyang lake,located in the northern part of Jiangxi province,is thelargest freshwater lake in China,it holds a great bulk of water from fiverivers:The Ganjiang,Fuhe,Xinjiang,Xiushui and Pojiang,and then passesthrough the Hukou(lake outlet)into Changjiang river.At the highest floodwater level it covers an area of about 4000km².According to investigation,the lake was originally the valley basin of the lower reach of ancient Ganjiang river.Later on,the lake water was heldback by the water of Changjiang river and its deposits occurred near Hukou,thus,held the flood from discharging rapidly and caused the area of flood extending southward.According to the historical records and excavationevidences,the fromation of Boyang lake and its deltaic development began about1500 years ago.The Boyang lake is a by-pass lake of inflow-outflow type,its deposi-tional mechanism is controlled by lake currents.When the lake level ishigher than that of the Changjiang river,the tractive current is the mainagency of the growing lakeward deltas.The average annual deposit volumeof the lake amounts to 11.2 million tons(7.5 million m³).According tothe area of the highest flood level,the average annual thick of depositsis about 2 mm.Since the Tang Dynasty(about 1200 years ago),theaverage annual depositional thick is about 2.5 mm in delta area,and up to 10 mm in local front zone of the deltas.Owing to the differences of hydrodynamic intensity and transportability of sand of the inflow rivers,and of land forms in the river mouths,thus,enable to the deltas to have some different development patterns,such asbird’s-foot extension,crevasse-distributaries,river mouth bar and subaque-ous delta,etc.The bird’s-foot extension associated with crevasse is themain pattern of delta.The characteristics of the depositional facies and the facies patterns of thelake and its deltas are as follows:There is a great water-level difference between the flood and dry period with an alternation of river and lake envi-ronment result in the lack of lacustrine deposit with wide area of stable dis-tribution;the tractive current of flowing character makes the front deltato develop a set of under water channels,under water levees,under water cre-vasse fans.interdistri butary bays and orther subfacies connecting with theland deltas;and simultaneously,the land deltaic plain developed to branched channels,levees,crevasse fans,interdistri butary depresions,abandoned cha-nnels-bayous and orther subfacies.In the vertical sequence,the lacustrinedeposits are overlain by delta deposits which resemble the river depositionalsequence of facies and it is difficult to distinguish the botton set,foresetand topset.This is clearly different from the classical delta of the deepwater lake.