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.
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 [
It has long been recognized that fluvial geomorphology concepts developed from the study of alluvial channels, while bedrock channels exhibit significant differences in hydrologic processes and evolution of geomorphic features [
Bedrock channels adjust to hydrologic change in a variety of ways. Incision can result downstream from landslide dams [
Channel avulsions play a major role in the geomorphic development of alluvial channels and floodplains, redistributing water and sediment across a broad spectrum of environments [
The Huron River (northern Ohio, USA.) consists of alternating alluvial and bedrock reaches, and has a documented recent history of avulsions occurring in the alluvial reaches, based upon historical aerial photographs between 1950-2015. The purpose of this paper is to determine the properties of the avulsion events in the alluvial reaches, as influenced by the upstream bedrock reaches. Our hypothesis is that some of the properties of the upstream bedrock reaches are “inherited” by the downstream alluvial reaches and result in consistent development of avulsion channels, including episodic re-occupation of certain avulsion sites.
The Huron River is approximately 96-km in length, flowing north across northern Ohio into Lake Erie, one of the Laurentian Great Lakes (
There is a US Geological Survey (USGS) gaging station located about 3 km downstream of the junction of the West and East branches (at RK 20.6). This
station (USGS 04199000) has continuous gage records from 1950-2018 except for a 7-year data gap between September 1980-October 1987. The gage is located in the town of Milan, Ohio, on a bridge pier of US Highway 250, at an elevation of 174-m∙msl [
The study area consists of alternating reaches of bedrock channels and alluvial channels (
that are transported downstream to form gravel bars with imbricated fabric. The stream bed often exhibits prominent jointing (
The alluvial reaches consist of alternating bar-and-pool topography with gravel medial, lateral, point, and confluent bars, with continuous alluvial cover on the stream bed (
Nine historical aerial photograph sets were obtained of the study area between 1950-2015. The sources for the aerial photographs include the United States Department of Agriculture, Natural Resources Conservation Service (USDA-NRCS), United States Geological Survey, Earth Resources Observation and Science Center (USGS-EROS), the Erie County Auditor’s Office (a local government agency), and the State of Ohio, Geographically Referenced Information Program (OGRIP). Characteristics of the images are summarized in
This study used ESRI ArcMAP version 10.3.1® as a GIS platform to overlay the georeferenced historical aerial photograph images on the 2015 DOQQ. Each image was georeferenced using a constellation of ground control points (GCPs) located at consistent features in all of the images, such as the geographical position of bridge abutments. Individual images were stretched or shrunk using a polynomial best-fit triangulation method to closely match the position of the GCPs on the 2015 DOQQ. The root mean square (RMS) error for these best-fits is given in
The properties of each channel avulsion “inherited” from the upstream bedrock channel reach include exit angles and exit distances. Exit angles were calculated by comparing the average azimuth of the upstream bedrock channel reach to the azimuth of the upstream part of the avulsion channel (i.e., where the avulsion channel leaves the main channel). The exit distance is the direct path distance between the downstream end of the bedrock channel reach and the upstream end of the avulsion channel.
Field work included ground-truthing GCPs, excavating trenches along stream banks, collecting vibracores, textural analysis of gravels in the field, and measuring joint sets. Laboratory work included sediment core analysis and sand sieve analysis for grain size. Both trenches and cores were used for stratigraphy and facies analysis.
Date Acquired | Source | Image Format | Scale | RMS Error (m) |
---|---|---|---|---|
21-Oct-1950 | USDA-NRCS | Black-and-White | 1:1320 | 0.57 |
16-May-1960 | USGS-EROS | Black-and-White | 1:60,000 | 0.81 |
22-Mar-1969 | USGS-EROS | Black-and-White | 1:24,000 | 1.62 |
29-Apr-1977 | USGS-EROS | Black-and-White | 1:80,000 | 1.06 |
20-Sep-1979 | USDA-NRCS | Black-and-White | 1:1320 | 1.22 |
9-Apr-1988 | USGS-EROS | Infrared | 1:40,000 | 1.50 |
15-Mar-1995 | USGS-EROS | Black-and-White | 1:12,000 | 1.35 |
1-Apr-2001 | Erie County Auditor Office | Black-and-White | 1:12,000 | 0.45 |
3-Jan-2015 | OGRIP | DOQQ | Base Image |
Explanation: USDA-NRCS is US Department of Agriculture, Natural Resources Conservation Service. USGS-EROS is US Geological Survey, Earth Resources Observation and Science Center. Erie County Auditor Office is a local government entity for tax assessment purposes. OGRIP is the Ohio Geographically Referenced Information Program (State of Ohio). DOQQ is a Digital Ortho Quarter Quadrangle image georectified to an existing statewide 2.5 foot DEM. The DOQQ served as the base image and previous images projected on to it (RMS error shown).
This study excavated 11 trenches at six locations representing the entrance and exit positions of avulsion channels that formed between 1950-2015. These were typically cutbank sites where the exposure was cleared and deepened to remove slopewash. The stratigraphy of each site was logged using Sedlog®. Gravel grain size analyses were conducted in the field by measuring dimensions of the short-, intermediate-, and long-axis of each clast using a caliper. Approximately 20 clasts were measured per gravel bed. The average grainsize was calculated as the average of the intermediate diameter of the 10 largest clasts. Sand samples were sieved in the laboratory.
Eight 7.6 cm-diameter vibracores were recovered in the vicinity of several avulsion channels, ranging from 75-cm to 147-cm in length. The aluminum core barrels of each core were split lengthwise, one half archived, and the other half cleaned, logged using Sedlog®, and sampled for sand grain size analysis. Sieve analysis followed ASTM protocol [
The trenches and vibracores were used for facies analysis. Twelve lithofacies were defined, using combinations of lithology, texture, and sedimentary structures (
Bedrock fractures in the bed of the Huron River occur in joint sets. The azimuths of individual fractures were measured at low-flow stage by wedging a thin board into the open fracture and then using a Brunton compass to measure the azimuth. The location of these measurements was determined using GPS. The joint data was imported into GIS and projected on the 2015 DOQQ at those locations. Finally, the composite data was plotted on a non-linear rose diagram
Code | Lithology | Textures | Sedimentary Structures | Interpretation |
---|---|---|---|---|
Gm | gravel | pebble-cobble | massive | gravel bar platform |
Gmi | gravel | pebble-cobble | massive, imbricated | gravel bar-head |
Smc | sand | coarse-grained | massive | suprabar platform |
Smf | sand | fine-grained | massive | proximal overbank |
St | sand | medium-grained | trough cross-bedded | dunes |
Sr | sand | fine-grained | ripple laminated | ripples |
Sl | sand | fine-grained | planar lamination | proximal overbank |
SSl | silt | coarse-grained | planar laminated | proximal overbank |
SSm | silt | coarse-grained | massive | proximal overbank |
Fl | mud | silt and clay | planar laminated | distal overbank |
Fm | mud | silt and clay | massive | distal overbank |
P | mixed | mixed | roots | modern/recent soil |
[
When the georeferenced aerial photographs from 1950, 1960, 1969, 1977, 1979, 1988, 1995, and 2001 are projected onto the 2015 georectified DOQQ, there are six channel avulsions that can be observed representing changes greater than the mean RMS error (ranging from 0.45 - 1.62 m). These avulsions occurred in the alluvial reaches between bedrock reaches (
Three channel avulsions occurred between the 1950 and 1960 historical aerial photographs. The position of these avulsion channels is shown in
One channel avulsion occurred between the 1969 and 1977 historical aerial photographs. The reach labelled C4 (
channel at C4 by 1979, and the former (western) avulsion channel was abandoned.
One channel avulsion occurred between the 1979 and 1988 historical aerial photographs. The reach labelled C5 (
One channel avulsion occurred between the 1995 and 2001 historical aerial photographs. The reach labelled C6 (
Hydrograph data from USGS gage station 04199000, located 3 km downstream of the study area, was used to match historical avulsions to known hydrologic events. The hydrograph (
On the other hand, there were numerous channel-forming hydrologic events during intervals of no new avulsions (1960-1969, 1977-1979, 1988-1995, and 2001-2015). In two cases, adjustments included abandonment of avulsion channels and re-establishment of previous main channels (1977-1979 and 2001-2015). The number of channel-forming events per year varies randomly from 0.11 to 0.50. There are no trends to the data set. It is evident that significant hydrologic events (measured by discharge and/or stage height) are necessary but not sufficient conditions for avulsion events on the Huron River.
Number of Avulsions | Interval between Aerial Photographs | Date of Channel-Forming Hydrologic Event | Peak Streamflow (m3∙s−1) | Stage Height (m) |
---|---|---|---|---|
3 | 1950-1960 | 11-Mar-1952 | 370 | 6.0 |
12-May-1956 | 510 | 6.4 | ||
22-Jan-1959 | 722 | 7.3 | ||
0 | 1960-1969 | 26-Apr-1961 | 381 | 6.0 |
12-Jul-1966 | 330 | 6.1 | ||
1 | 1969-1977 | 5-Jul-1969 | 1389 | 9.5 |
17-Feb-1976 | 316 | 6.2 | ||
0 | 1977-1979 | 15-Dec-1977 | 325 | 6.3 |
1 | 1979-1988 | 14-Apr-1979 | 269 | 5.9 |
0 | 1988-1995 | 13-Nov-1992 | 294 | 6.1 |
1 | 1995-2001 | 27-Feb-1997 | 345 | 6.3 |
26-Aug-1998 | 470 | 7.1 | ||
0 | 2001-2015 | 22-Jun-2006 | 400 | 7.3 |
20-Aug-2007 | 356 | 6.9 | ||
7-Feb-2008 | 328 | 6.7 | ||
9-Mar-2009 | 468 | 6.7 | ||
28-Feb-2011 | 666 | 7.5 | ||
10-Jul-2013 | 490 | 6.9 | ||
22-Dec-2013 | 468 | 6.8 |
Explanation: “Channel-forming hydrologic events” are defined as discharge > 300 m3∙s−1 and/or stage height > 6 m. Data from USGS gage station 04199000 on the Huron River at Milan, Ohio.
Trenches and cores were used to assess the conditions of current and abandoned avulsion channels. The avulsion channels all occur in the alluvial reaches. These portions of the Huron River are composed of fine-grained alluvium banks generally < 2-m tall. The primary controls over channel incision and widening are plant roots (
Infilling of abandoned channels produced a characteristic two-component vertical sequence. Trenches and vibracores reached bedrock at the base of most abandoned channels. Bedrock was overlain by a variety of coarse-grained deposits. These typically consisted of massive gravels (lithofacies Gm) and imbricated gravels (lithofacies Gmi). Lithofacies Gm is interpreted as gravel bar platform deposits and lithofacies Gmi is interpreted as bar-head deposits [
The lower coarse-grained sequence is abruptly overlain by significantly finer-grained deposits (
Although this study examined recent historical changes in the Huron River (constrained by the oldest aerial photograph being from 1950), there was evidence in some trenches and cores of multiple avulsions occurring at the same location. For example, trenches and vibracores from avulsion channels C1 and C2 both revealed multistory sequences consisting of at least two channel occupation and abandonment successions. This suggests that episodic re-occupation of avulsion channels occurs at specific sites.
Sinuosity (thalweg path length/direct path length) varied from 1.65 to 1.77 for the study area between 1950-2015. Sinuosity changes for the Huron River are clearly linked to the occurrence of channel avulsions (
1995-2001, the sinuosity decrease from 1.73 to 1.67 corresponded to avulsion C6. Finally, between 2001-2015, the sinuosity increase from 1.67 to 1.74 corresponded to no avulsions.
The limits of the dataset are that sinuosity is calculated for the time interval when historical aerial photographs were recorded, but the avulsions occurred in the time interval between one photograph and the next one. Thus the sinuosity values given above (
Prior to Avulsion(s) | After Avulsion(s) | |||
---|---|---|---|---|
Date | Sinuosity | Avulsion(s) | Date | Sinuosity |
21-Oct-1950 | 1.77 | 3 | 16-May-1960 | 1.70 |
16-May-1960 | 1.70 | 0 | 22-Mar-1969 | 1.73 |
22-Mar-1969 | 1.73 | 1 | 29-Apr-1977 | 1.71 |
29-Apr-1977 | 1.71 | 0 | 20-Sep-1979 | 1.72 |
20-Sep-1979 | 1.72 | 1 | 9-Apr-1988 | 1.66 |
9-Apr-1988 | 1.66 | 0 | 15-Mar-1995 | 1.73 |
15-Mar-1995 | 1.73 | 1 | 1-Apr-2001 | 1.67 |
1-Apr-2001 | 1.67 | 0 | 3-Jan-2015 | 1.74 |
Explanation: sinuosity = channel thalweg path length/valley path length.
sinuosity increased between avulsion events to values of P ≥ 1.72, and then at some time after this (and possibly at a higher value of P), one or more avulsions occurred.
Bedrock fractures occurred in sets of parallel orientations (joint sets). Because the fractures were close to vertical, this study focused on the azimuth representing the strike of the individual fracture. In the field, efforts were made to avoid measuring the same fracture repeatedly. The data is summarized as a rose diagram (
The orientation of the channel in bedrock reaches closely corresponds to the orientation of joint sets in the underlying bedrock. In
Properties of upstream bedrock reaches appear to be transferred to downstream alluvial reaches and control the location and directions of avulsion channels. As shown in
Avulsion Channel Identifier | Azimuth of Upstream Bedrock Channel Reach | Azimuth of Downstream Avulsion Channel Reach | Exit Angle (degrees) | Exit Distance (m) |
---|---|---|---|---|
C1 | 340 | 342 | 2 | 101 |
C2 | 080 | 070 | 10 | 116 |
C3 | 340 | 338 | 2 | 142 |
C4 | 329 | 344 | 15 | 266 |
C5 | 340 | 018 | 38 | 175 |
C6 | 043 | 048 | 5 | 62 |
Mean | 12.1˚ | 143.7 m | ||
Standard deviation | 13.7˚ | 71.0 m |
Explanations: Exit angle = deviation between azimuth of upstream bedrock reach and downstream avulsion reach. Exit distance = distance between exit of bedrock reach and upstream end of avulsion channel.
underlying joint sets control the direction of both bedrock and alluvial channels in the Huron River, although bedrock is not exposed in the alluvial channels. Alternatively, the flow exiting the bedrock reaches is directed toward the banks of the alluvial reaches, erodes the bank materials, and influences the sequential development of the avulsion channel.
The position of the avulsion channel ranges from 62 - 266 m downstream of the exit from the upstream bedrock channel, with a mean and standard deviation of 143.7 ± 71.0 m. This distance does not appear to be controlled by meander dynamics in the alluvial reaches (meander wavelength is about 1.2 km). There are no natural or human-constructed obstructions or obstacles that would explain localization of bank erosion and avulsion channel development within such a relatively short distance after the flow “exits” the upstream bedrock channel.
It is suggested that the bedrock channel, strongly aligned by underlying joint set orientations, directs the flow into the downstream alluvial reach, establishing the location and orientation for incipient avulsion channel development, hence these are “inherited” characteristics. Support for this idea is provided by evidence that some of the avulsion channel sites were locations of multiple, episodic avulsion events. Alternatively, it is possible that underlying bedrock fractures are somehow controlling flow directions in the alluvial reaches, even though bedrock is not exposed. It is also possible that avulsion channels develop repeatedly at some sites because the avulsion channel fills are somehow less resistant to subsequent bank erosion than bank materials elsewhere. Avulsion channel-fill sequences have coarse-grained materials at the base, but the upper parts are fine-grained sediment identical to floodplain deposits observed elsewhere in the study area.
Bedrock channel reaches in the Huron River show active erosion processes, such as mass wasting of the exposed bedrock cliffs where rockfall debris goes directly into the channel, or places with active hydraulic quarrying of bedrock slabs from the channel bottom. There is little alluvial storage in the channel, and where it happens it is typically pool-fills or small pendant bars. Thus the bedrock channel reaches are considered erosional or transport reaches. Alluvial channel reaches in the Huron River show a variety of medial, lateral, point, and confluence gravel bars [
The Huron River demonstrates the complex interactions of rivers with alternating bedrock channel reaches and alluvial channel reaches. As expected, the bedrock channels are erosional or transport reaches with little long-term sediment accumulation and the alluvial channels are depositional reaches. Also as expected, the bedrock channels are strongly influenced by characteristics of the underlying bedrock, in this case the orientation of joint sets, and there is little evidence for channel adjustment in these bedrock reaches. What was not expected in this study was evidence for downstream influence of the bedrock channel reaches, such as the location (“exit distance”), orientation of avulsion channels, and repeated occupation of certain avulsion sites―what we term “inherited characteristics”. 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.
As noted by previous workers, an avulsion is triggered by a hydrologic event, in this case flows greater than the 5-year recurrence interval (P = 0.20), which represents channel-forming discharge events. However, because not every such event resulted in an avulsion during this 65-year study interval, such flows are necessary but not sufficient conditions. The more important variable appears to be sinuosity, which increased from P = 1.65 to P = 1.72 - 1.77 prior to triggering an avulsion event. This suggests that the “critical sinuosity” [
We wish to thank Jocelyn Hicks, Lyere Okojie, Allan Adams, and students of the BGSU Surface Water Hydrology class for their assistance in the field. The Ohio Geological Survey provided use of the vibracorer. A portion of this study was a Master’s degree thesis project by Potucek. Funding was provided by the BGSU Department of Geology and Mancuso Family Field Scholarship.
The authors declare no conflicts of interest regarding the publication of this paper.
Potucek, M.J. and Evans, J.E. (2019) Avulsion Dynamics in a River with Alternating Bedrock and Alluvial Reaches, Huron River, Northern Ohio (USA). Open Journal of Modern Hydrology, 9, 20-39. https://doi.org/10.4236/ojmh.2019.91002