11:00 AM - 1:00 PM
[HGM03-P06] Impacts of bedrock river meandering on channel gradient
Keywords:Bedrock river, Meander, Shimanto belt
Bedrock river morphology reflects various geologic and climatic factors, and disentangling their relations help to reveal the long-term landscape evolution and infer rates of erosion and uplift by analyzing river-long profiles. While many have studied how bedrock river profiles respond to environmental conditions such as tectonics, less attention has been paid to changes in channel length associated with meandering and its influence on bedrock river forms. Changes in channel length directly affect channel gradient, and it is important to understand the dynamics of meandering in bedrock rivers when interpreting longitudinal river profiles. For example, channel lengthening and shortening associated with meander growth and cutoffs modify channel gradient, potentially leading to a decrease or increase in vertical incision rates. If the changes in channel length continue, local incision rates of the meandering section are expected to divert from rates in its neighboring sections, potentially affecting the gradient of immediately upstream and downstream of the meandering section. Therefore, the influence of meander growth is not necessarily limited in the meandering sections, and the evolution of river-long profiles associated with meander growth may not be simple. This study examines changes in bedrock channel gradient due to meandering and aims to quantify the impact of meandering on channel gradient.
I focused on rivers flowing over the Shimanto Belt (Fig. 1), where highly sinuous rivers occur. I first extracted stream networks using 10 m DEM obtained from the Geospatial Information Authority of Japan. Upstream limits of the streams are defined based on a minimum drainage area of 1 km2, and downstream limits are placed at the valley mouth, presumably corresponding to the boundary between a bedrock river and an alluvial plain. Next, I divided the streams into segments of 1 km in streamwise length and calculated average normalized channel steepness and a sinuosity index for each segment. Normalized channel steepness (ksn) is an index of channel gradient calculated using channel gradient and drainage area, and higher values indicate steeper channels. A sinuosity index is a streamwise length divided by the Euclidean distance between the upstream and downstream ends of the segment. Although river incision rates are necessary to investigate the impact of meandering on ksn, for convenience, I used topographic relief as a substitute because the number of available incision rate data is limited, and topographic relief tends to correlate with incision rates positively. Topographic relief is defined as the elevation difference between the highest and lowest points within an area 2 km from the river segments. Preliminary results indicated that a sinuosity index negatively correlated with ksn when relief was comparable. If topographic relief correctly represents incision rates in the study areas, the current results suggest channel lengthening due to meander growth may significantly decrease channel gradient. However, further analysis is needed to discuss a causal relationship between sinuosity indices and ksn because gradient changes may occur both in the meandering section and its neighboring sections, and there are many other factors that influence channel gradient.
I focused on rivers flowing over the Shimanto Belt (Fig. 1), where highly sinuous rivers occur. I first extracted stream networks using 10 m DEM obtained from the Geospatial Information Authority of Japan. Upstream limits of the streams are defined based on a minimum drainage area of 1 km2, and downstream limits are placed at the valley mouth, presumably corresponding to the boundary between a bedrock river and an alluvial plain. Next, I divided the streams into segments of 1 km in streamwise length and calculated average normalized channel steepness and a sinuosity index for each segment. Normalized channel steepness (ksn) is an index of channel gradient calculated using channel gradient and drainage area, and higher values indicate steeper channels. A sinuosity index is a streamwise length divided by the Euclidean distance between the upstream and downstream ends of the segment. Although river incision rates are necessary to investigate the impact of meandering on ksn, for convenience, I used topographic relief as a substitute because the number of available incision rate data is limited, and topographic relief tends to correlate with incision rates positively. Topographic relief is defined as the elevation difference between the highest and lowest points within an area 2 km from the river segments. Preliminary results indicated that a sinuosity index negatively correlated with ksn when relief was comparable. If topographic relief correctly represents incision rates in the study areas, the current results suggest channel lengthening due to meander growth may significantly decrease channel gradient. However, further analysis is needed to discuss a causal relationship between sinuosity indices and ksn because gradient changes may occur both in the meandering section and its neighboring sections, and there are many other factors that influence channel gradient.