Impacts of flow regulation on geomorphic adjustment and riparian vegetation succession along an anabranching reach of the Upper Yellow River

Abstract. The influence of vegetation upon bedload transport and channel morphodynamics is examined along a channel stability gradient ranging from meandering to anabranching to anabranching–braided to fully braided planform conditions along trunk and tributary reaches of the Upper Yellow River in western China. Although the regional geology and climate are relatively consistent across the study area, there is a distinct gradient in the presence and abundance of riparian vegetation for these reaches atop the Qinghai–Tibet Plateau (elevations in the study area range from 2800 to 3400 m a.s.l.). To date, the influence of vegetative impacts upon channel planform and bedload transport capacity of alluvial reaches of the Upper Yellow River remains unclear because of a lack of hydrological and field data. In this region, the types and pattern of riparian vegetation vary with planform type as follows: trees exert the strongest influence in the anabranching reach, the meandering reach flows through meadow vegetation, the anabranching–braided reach has a grass, herb, and sparse shrub cover, and the braided reach has no riparian vegetation. A non-linear relation between vegetative cover on the valley floor and bedload transport capacity is evident, wherein bedload transport capacity is the highest for the anabranching reach, roughly followed by the anabranching–braided, braided, and meandering reaches. The relationship between the bedload transport capacity of a reach and sediment supply from upstream exerts a significant influence upon channel stability. Bedload transport capacity during the flood season (June–September) in the braided reach is much less than the rate of sediment supply, inducing bed aggradation and dynamic channel adjustments. Rates of channel adjustment are less pronounced for the anabranching–braided and anabranching reaches, while the meandering reach is relatively stable (i.e., this is a passive meandering reach).

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Channel geomorphology and riparian vegetation interactions along four anabranching reaches of the Upper Yellow River

Progress in Physical Geography Earth and Environment ◽

10.1177/0309133320938768 ◽

2020 ◽

Vol 44 (6) ◽

pp. 898-922

Author(s):

Meiqin Han ◽

Gary Brierley

Keyword(s):

Riparian Vegetation ◽

Yellow River ◽

Distribution Patterns ◽

Tree Cover ◽

Primary Control ◽

Pioneer Plant ◽

Geomorphic Features ◽

Notable Increase ◽

Upper Yellow River ◽

Shrub Patches

The fluvial biogeomorphic succession (FBS) phase model is used to appraise biogeomorphic interactions along four anabranching reaches of the Upper Yellow River. Geomorphic features and vegetation distribution patterns are combined to assess the dominant FBS phase. A notable increase in shrub and tree cover and a decrease in sedges occur with decreasing elevation from 4200 to 2200 m asl. The Maduo reach (4185 m) is characterized by the biogeomorphic phase, while the Dari (3960 m) reach is at the pioneer phase, with short shrubs playing a prominent role. Both the Maqu (3450 m) and the Guide (2117 m) reaches are ecological phase dominated (the Guide reach has been impacted by flow regulation in recent decades). Reaches other than the Maduo have experienced an increase in post-pioneer vegetation (woody plants) in recent decades, with a decrease in pioneer plant communities (grass and sedges). The Dari reach experienced the most dynamic channel planform changes, with a marked increase in short shrub patches from 2007 to 2017. An increase in woody vegetation underpinned increasingly stable geomorphic dynamics in the Maqu reach. Human activities have restricted geomorphic impacts of expanded areas of woody riparian vegetation in the Guide reach. Variability in dominant FBS phases exerts a primary control upon channel behavior and evolutionary traits in the four study reaches.

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