Abstract
The Huron River consists of alternating bedrock reaches and alluvial reaches. Analysis of historical aerial photography from 1950-2015 reveals six major channel avulsion events in the 8-km study area. These avulsions occurred in the alluvial reaches but were strongly influenced by the properties of the upstream bedrock reach (“inherited characteristics”). The bedrock reaches aligned with the azimuth of joint sets in the underlying bedrock. One inherited characteristic in the alluvial reach downstream is that the avulsion channels diverged only slightly from the orientation of the upstream bedrock channel (range 2 ° - 38 °, mean and standard deviation 12.1 ° ± 13.7 °). A second inherited characteristic is that avulsion channels were initiated from short distances downstream after exiting the upstream bedrock channel reach (range 62 - 266 m, mean and standard deviation 143.7 ± 71.0 m), which is a fraction of the meander wavelength (1.2 km). Field evidence shows that some avulsion channel sites were re-occupied episodically. In addition, two properties were necessary for channel avulsions: 1) avulsion events were triggered by channel-forming hydrologic events (5-year recurrence interval flows), but not every channel-forming hydrologic event resulted in an avulsion, and 2) channel sinuosity (P) increased to 1.72 - 1.77 prior to an avulsion then decreased to 1.65 - 1.70 following an avulsion, suggesting that P ≥ 1.72 is the “critical sinuosity” or triggering value for avulsions on the Huron River. In summary, for this river consisting of alternating bedrock and alluvial reaches, the bedrock reaches impose certain parameters on downstream alluvial reaches (including sediment supply, channel direction and avulsion channel position downstream after exiting a bedrock reach) while adjustments in sinuosity and sediment storage occur in the alluvial reaches.
Highlights
This study evaluates a river that consists of alternating alluvial and bedrock reaches
The Huron River demonstrates the complex interactions of rivers with alternating bedrock channel reaches and alluvial channel reaches
These features may be due to bedrock channels steering the flow toward the banks of the downstream alluvial channels, alternatively bedrock may be exerting control in the alluvial reaches even though it is not exposed in the channel bed materials
Summary
This study evaluates a river that consists of alternating alluvial and bedrock reaches. An alluvial channel is understood to have bed and bank materials that consist of loose sediment, possibly including gravel. A bedrock channel has been variously defined as having bed or bank materials that are >50% lithified rocks or cemented alluvium [1] or, alternatively, channel beds that lack even a thin continuous alluvial cover [2] [3]. The West Branch is 80-km in length, and this study examines the 8-km reach just upstream of the junction of the East and West branches (between river kilometers (RK) 28 - 36). The West Branch is found in a hilly region called the Allegheny Escarpment, which consists of bedrock hills having topographic relief up to 175-m above the bed of the adjacent stream
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