5:15 PM - 6:45 PM
[ACG43-P03] Contrasting grain size evolution in headwaters controlled by rock strength and provenance
Keywords:Grain size
The mobility of sediment particles depends on grain sizes, suggesting the downstream changes in grain sizes play an important role in the evolution of the fluvial landscape. Grain sizes typically decrease downstream due to abrasion, breaking, and selective transport. Various factors, such as the durability of particles and the grain size distribution of bed materials, control the rates of downstream fining. Supply of coarse materials from tributaries and by mass movements can locally alter the fining rates and sometimes cause downstream coarsening. When the bed materials consist of multiple rock types, each rock type experiences the factors mentioned above at different magnitudes, resulting in the complex evolution of grain size. Therefore, to understand the causes of the downstream evolution of grain size, we need to reveal how changes in sediment sources and the mixing of different rock types affect the grain size distribution. In this presentation, we show an example where sizes of basalt and shale gravels exhibit an opposite downstream evolution in the headwaters of mountain rivers and discuss the factors that caused the contrasting grain size evolution.
We focused on the ~4 km section of the upstream part of Mosawa located in Tsugaru Mountain, northern Japan. In this section, basalt and dolerite are exposed in the upstream, while shale is dominant in the downstream part. At 15 sites along the trunk stream, we randomly picked gravels from the surface of gravel bars and measured their intermediate axis. In cases where the number of shale gravels measured was less than 100, we continued to measure only shale gravels until we measured more than 100 grains.
The median grain size (D50) of basalt decreased downstream at approximately 6% per kilometer. The fining rates were greater for coarse-grained components: D14 did not change significantly, while D84 decreased at 9% per kilometer. Because the supply of basaltic grains from hillslopes is limited to the upstream part of the studied section, the observed difference in the fining rates probably results from the limited supply of coarse grains. D50 of shale remained relatively stable in the upstream part where shale was partially exposed. As the shale outcrops more extensively in the downstream part, D50 increased from 2 to 6 cm. The sizes of shale gravels can rapidly decrease due to slaking. Also, because of their small size (D50 < 6 cm), the residence time of shale gravels in the channel is probably short. Thus, the total amount of shale gravel at the channel bottom is relatively small, increasing the influence of coarse material supplied from hillslopes on the grain sizes in the channel. To examine if the mixture of different rock types alters the downstream evolution of grain size, we also measured grain sizes in a tributary composed entirely of shale.
We found that the shale gravels in the tributary were smaller than those in the trunk stream. Since shale is constantly supplied to the tributaries, if the mixing of basalt and shale does not affect the downstream changes in grain size, it is expected that the grain size in the tributary will be larger or similar to that in the trunk stream. Thus, we interpret that the difference in the size of shale gravels between the trunk stream and tributaries is related to the presence of basalt gravels in the trunk stream. Basalt gravels are much less susceptible to slaking than shale. When shale and basalt gravels coexist in a gravel bar, shale gravels deposited over basalt gravels will experience wet-dry cycles less frequently compared to shale gravels at the lower part of the bar or in a bar containing only shale. As a result, shale gravels sitting on basalt gravels can survive fining due to slaking for a longer period than those in a bar abundant in shale. Our results indicate significant variations in particle size trends by rock type due to differences in durability against processes that decrease particle size and geological distribution within the basin.
We focused on the ~4 km section of the upstream part of Mosawa located in Tsugaru Mountain, northern Japan. In this section, basalt and dolerite are exposed in the upstream, while shale is dominant in the downstream part. At 15 sites along the trunk stream, we randomly picked gravels from the surface of gravel bars and measured their intermediate axis. In cases where the number of shale gravels measured was less than 100, we continued to measure only shale gravels until we measured more than 100 grains.
The median grain size (D50) of basalt decreased downstream at approximately 6% per kilometer. The fining rates were greater for coarse-grained components: D14 did not change significantly, while D84 decreased at 9% per kilometer. Because the supply of basaltic grains from hillslopes is limited to the upstream part of the studied section, the observed difference in the fining rates probably results from the limited supply of coarse grains. D50 of shale remained relatively stable in the upstream part where shale was partially exposed. As the shale outcrops more extensively in the downstream part, D50 increased from 2 to 6 cm. The sizes of shale gravels can rapidly decrease due to slaking. Also, because of their small size (D50 < 6 cm), the residence time of shale gravels in the channel is probably short. Thus, the total amount of shale gravel at the channel bottom is relatively small, increasing the influence of coarse material supplied from hillslopes on the grain sizes in the channel. To examine if the mixture of different rock types alters the downstream evolution of grain size, we also measured grain sizes in a tributary composed entirely of shale.
We found that the shale gravels in the tributary were smaller than those in the trunk stream. Since shale is constantly supplied to the tributaries, if the mixing of basalt and shale does not affect the downstream changes in grain size, it is expected that the grain size in the tributary will be larger or similar to that in the trunk stream. Thus, we interpret that the difference in the size of shale gravels between the trunk stream and tributaries is related to the presence of basalt gravels in the trunk stream. Basalt gravels are much less susceptible to slaking than shale. When shale and basalt gravels coexist in a gravel bar, shale gravels deposited over basalt gravels will experience wet-dry cycles less frequently compared to shale gravels at the lower part of the bar or in a bar containing only shale. As a result, shale gravels sitting on basalt gravels can survive fining due to slaking for a longer period than those in a bar abundant in shale. Our results indicate significant variations in particle size trends by rock type due to differences in durability against processes that decrease particle size and geological distribution within the basin.