10:45 AM - 12:15 PM
[HCG22-P02] Does sediment dynamics affect a link between rock strength and channel gradient?
Keywords:Bedrock river, Rock strength, Sediment dynamics, Tsugaru Mountains
A longitudinal gradient of bedrock rivers correlates with substrate rock strength. All else being equal, bedrock rivers tend to get steeper when incising into stronger rocks. This is because more energy is required to erode stronger rocks at the same rate as weaker rocks when sediment transport capacity far exceeds sediment supply. However, the connection between rock strength and channel gradient becomes unclear when sediment supply exceeds transport capacity. Bed load enhances bedrock incision by particle impact and inhibits incision by armoring the bed, playing a pivotal role in determining the efficiency of bedrock incision. We speculate that the difference in rock strength may be compensated by the contrast of erosional efficiency, resulting in a uniform channel gradient across a lithologic boundary. This study aims to test this hypothesis based on DEM analysis and field observation of bedrock rivers in Tsugaru mountains, Aomori.
We focused on rivers around Hakamagoshi-dake, a dome mainly composed of basalt and dolerite. Neogene sedimentary rocks occur downstream of the studied rivers. The river reaches underlain by igneous rocks have slopes 1-3 times greater than those underlain by sedimentary rocks. This observation implies that the difference in rock strength may be strongly reflected in channel gradients in some rivers. We measured channel width, depth, and grain size and recorded a fraction of exposed bedrock at the bed. Then, we calculated the minimum channel gradient required to entrain sediments at surveyed points and those supplied from upstream. Also, we calculated a ratio of sediment supply to transport capacity and a transport stage, a ratio of the Shields number to the critical Shields number. These two ratios are often used to assess erosional efficiency by bed load. We used normalized channel steepness (ksn) to facilitate the comparison of channel gradient between points of different catchment areas.
We conducted a field survey in three rivers at the western flank of Tsugaru mountains (Tano-sawa, Ohkura-sawa, and Shikiba-sawa). Basaltic rocks dominate the studied reaches in Tano-sawa. Its trunk stream has two times larger ksn than its tributary. The median grain size (D50) of the trunk stream is 1.5 times larger than that of the tributary. This difference in D50 is responsible for the observed difference in ksn. A similar correlation between ksn and D50 is found in Ohkura-sawa, where ksn and D50 are much larger in the reaches of basaltic rocks than in the reaches of sandstone and mudstone. These results confirm that grain size rather than rock resistance to erosion primarily controls the gradient of bedrock rivers, as previously suggested by numerical modeling and field observations elsewhere. In contrast, in Shikiba-sawa, ksn and D50 are indistinguishable between the reaches of basaltic rocks and sedimentary rocks. The ratio of sediment supply and transport capacity is higher in the reaches of sedimentary rocks. The transport stage is similar in both reaches. These results may indicate that erosional efficiency is probably different. However, further observations and comparisons to the result of numerical modeling are necessary to quantify the competing effects of rock strength and bed load sediment on channel gradient.
We focused on rivers around Hakamagoshi-dake, a dome mainly composed of basalt and dolerite. Neogene sedimentary rocks occur downstream of the studied rivers. The river reaches underlain by igneous rocks have slopes 1-3 times greater than those underlain by sedimentary rocks. This observation implies that the difference in rock strength may be strongly reflected in channel gradients in some rivers. We measured channel width, depth, and grain size and recorded a fraction of exposed bedrock at the bed. Then, we calculated the minimum channel gradient required to entrain sediments at surveyed points and those supplied from upstream. Also, we calculated a ratio of sediment supply to transport capacity and a transport stage, a ratio of the Shields number to the critical Shields number. These two ratios are often used to assess erosional efficiency by bed load. We used normalized channel steepness (ksn) to facilitate the comparison of channel gradient between points of different catchment areas.
We conducted a field survey in three rivers at the western flank of Tsugaru mountains (Tano-sawa, Ohkura-sawa, and Shikiba-sawa). Basaltic rocks dominate the studied reaches in Tano-sawa. Its trunk stream has two times larger ksn than its tributary. The median grain size (D50) of the trunk stream is 1.5 times larger than that of the tributary. This difference in D50 is responsible for the observed difference in ksn. A similar correlation between ksn and D50 is found in Ohkura-sawa, where ksn and D50 are much larger in the reaches of basaltic rocks than in the reaches of sandstone and mudstone. These results confirm that grain size rather than rock resistance to erosion primarily controls the gradient of bedrock rivers, as previously suggested by numerical modeling and field observations elsewhere. In contrast, in Shikiba-sawa, ksn and D50 are indistinguishable between the reaches of basaltic rocks and sedimentary rocks. The ratio of sediment supply and transport capacity is higher in the reaches of sedimentary rocks. The transport stage is similar in both reaches. These results may indicate that erosional efficiency is probably different. However, further observations and comparisons to the result of numerical modeling are necessary to quantify the competing effects of rock strength and bed load sediment on channel gradient.