Landscape connectivity and sediment flux within the upper Yellow River basin

Abstract

The degree of landscape connectivity has wide-ranging implications for sediment availability, frequency of transport, and the nature of sediment storage within a basin. Looking at the system as a whole, and identifying the connections that facilitate or impede sediment movement within a catchment is central to these applications. This thesis examines landscape connectivity within the highly diverse landscapes of the upper Yellow River (UYR) basin at a broad scale, with detailed focus on a smaller tributary that lies in the incised basin fill deposits of the Guide basin close to the margin of the Qinghai-Tibetan Plateau (QTP), the Garang subcatchment. Uplift of the QTP has resulted in a high-altitude landscape with a cold, semi-arid continental climate within the upper Yellow River. The region is characterized by several wide, low-relief basins separated by the two major mountain ranges (up to 2 km in relief) that run through the region, with tectonic deformation enduring within a series of strike-slip fault complexes. The thesis results are presented as a series of three papers. Findings are brought together in a discussion chapter. The first paper focuses on the pronounced variability in the landscapes of the upper Yellow River basin. The classification presented in this paper provides an effective organizational framework to describe the landscape diversity. Stark contrasts in landform assemblages and associated process relationships are evident across three very different terrains, reflecting the complex inter-relationships between tectonics, climate and surficial processes over time. A broad, low-relief, and highly disconnected upper plateau area at the headwaters of the UYR represents a relict peneplain that may have formed prior to regional uplift. The ranges of the Anyemaqen Shan in the central basin form a high-relief and highly connected landscape. Finally, the incisional story of the UYR dominates within the lower portion of the study area, where low-relief basin fills have been highly incised as a result of headward erosion of the Yellow River as drainage was established through the area. The second and third papers present a detailed examination of the landscape connectivity and sediment dynamics within the Garang study catchment. The second paper applies two methodological approaches for assessing landscape connectivity, a GIS-based geomorphometric index and a methodology linking interpretation of satellite imagery and field mapping of sediment storage to slope threshold analysis. Landscapes of the Garang catchment are differentiated into three geomorphic zones characterized by distinct landscape configuration and dominant geomorphic processes: i) a highly disconnected upper catchment of low-relief with large inactive sediment stores; ii) a transitional zone where present landscape dynamics are controlled in large part by past incisional processes in the form of large alluvial fan/terrace deposits; and iii) a highly connected and highly dissected landscape within the lower catchment that has little accommodation space for sediment storage. The findings from this paper emphasize the need for field-based observations that are capable of differentiating between landforms and activity levels of sediment stores, as well as providing inference on geomorphic process, that may not be evident with the use of cell-based morphometrics. The final paper expands upon these findings and presents an overview of sediment distribution and volume within the highly incised Garang catchment, combining field and GIS-based analyses. The magnitude and pattern of sediment storage is shown to be highly disparate between three distinct geomorphic zones of the Garang catchment. Findings of the study also reveal a somewhat unconventional pattern of sediment storage, whereby sediment storage is greater within the headwaters and decreases with distance downstream, adding to the range of landscape settings in which catchment-scale patterns of sediment storage have been assessed. The study also provides insight into the influence of long-term landscape evolution within the area, and how the response to lowering of the base level through Yellow River incision has impacted landscape connectivity and associated patterns of sediment storage and reworking within the catchment. Findings from both studies highlight the importance of field-informed appraisals of landscape dynamics, site-specific characteristics and the significance that basin-scale history can have on determining contemporary sediment dynamics. Issues associated with scales of analysis and the importance of localized influences are a key theme within the thesis. The final discussion chapter contextualizes findings of the thesis, focusing primarily on scale relations between landforms, geomorphic compartments (zones) and the subcatchment-scale analysis, and prospects to meaningfully up-scale these understandings to the UYR as a whole, linking analyses at the subcatchment scale to considerations of how we approach connectivity analyses across differing scales and contexts. Limitations and implications of the study are outlined.