Fluvial Processes And Wetland Degradation Mechanism Of The Sanjiangyuan Source

Sanjiangyuan (Yangtze River, Yellow River and Langcang River) provides anideal observed window and experiment sites on fluvial processes of meandering rivers,anabranching rivers and braided rivers, and wetland research without human activitydisturbance, which is rich in landscape diversity of fluvial geomorphology in thebackground of Qinghai-Tibet Plateau. Modern fluvial processes within theSanjiangyuan source include scientific problems of meandering, anabranching andbraided river of their trunk stream and tributaries. Meandering, anabranching andbraided rivers are the dominant river pattern of the source, where basic researchproblems are strengthened and of great significance to enrich the contents of fluvialprocesses. The Yellow river main stream, Baihe and Heihe Rivers incise throughout theRuoergai (Zoige) basin from three directions. Rapid shrinkage and degradation of theRuoergai swamp closely links with the river processes of riverbed incision, rivernetwork headcut erosion and artificial excavated ditches. Only quantitatively graspingthe contribution of natural rivers and artificial ditches on the Ruoergai swampshrinkage, the long-term and short-term shrinkage mechanism can be truly revealed.According to field investigation and experiments of four years in2010-2013,meteorological and hydrological data and remote sensing images, fluvial processes ofthe Sanjiangyuan, morphological characteristics and cutoff mechanism, developmentpattern and degradation mechanism of sandbars were deeply analyzed, the long-termdegradation causes and short-term accelerated shrinkage of the Ruoergai swamp arerevealed, and the protection and restoring methods of degraded swamp are put forward.The main drainage of Yangtze River source has parallel drainage pattern and belongsto dendritic river network, where braided rivers are extensively developed. The riverpattern of the Yellow River source within the First Great Bend apperars four times ofriver patterns spatial transition phenomena along270km stream course, which areanastomosing-anabranching, anastomosing-meandering, meandering-braided andbraided-meandering pattern transition. Neck cutoffs of meandering rivers are classifiedinto bank collapsing, rush cutting and erosion ditch pattern. Chute cutoffs can be furthersorted into shoal cutting erosion pattern, erosion ditch scouring pattern and main flow scouring embayment pattern. Therefore, the occurrence mechanisms of neck cutoffs andchute cutoffs are obviously different. Suspended sediment concentration andsediment deposition length are two key factors to determine the formed rate ofoxbow lakes after neck cutoffs. Sandbars of alluvial channel generally growupstream by bar head deposition. Four sets experiments of bar deposition and erosion,the least resistance deduced along the course, vegetation coverage and particle sizedistribution of bar surface, the critical incipient motion and the flow carryingsediment capacity analysis are used to verify the existence of the dominant mode, whichis bar head deoposition and upstreamward development due to the superposition effectof bar head blocking and branching.The influence of climate warming is quite limited on the shrinkage of the Ruoergaiwetland and therefore, not the main reason of the wetland degradation, especiallyconsidering seasonal characterization of wetland appearance. Artificial ditch excavationin1960-1970s as an intense disturbance of human activities is a key reason on rapidshrinkage of wetland area last50decades. Artificial excavated ditches have three waysto accelerate the Ruoergai wetland shrinking: direct drainaging water of closed wetland,strengthening connectivity of natural drainage and artificial ditches, and downcuttingthe peat layer accelerating the rate of headcut erosion. Riverbed incision of YellowRiver, Bai River, Hei River and their tributaries is an underlying reason of the long-termnatural degradation of the wetland. Headcut incision of countless tributaries inside theswamp can continuously drain out water, slowly lower groundwater water level and cutinto the inside of wetland, which is the most important mechanism of the wetlanddegradation.