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Using Pre-existing Channel Substrate to Determine the Effectiveness of Best Management Practices, Sandusky River, Ohio
J. Great Lakes Res. 33 (Special Issue 2):167–181 Internat. Assoc. Great Lakes Res., 2007
Using Pre-existing Channel Substrate to Determine the Effectiveness of Best Management Practices, Sandusky River, Ohio Ryan P. Murphy1, Enrique Gomezdelcampo2,*, and James E. Evans2 1Hull
& Associates, Inc. 3401 Glendale Avenue, Suite 300 Toledo, Ohio 43614 2School
of Earth, Environment and Society Bowling Green State University Bowling Green, Ohio 43403-0211
ABSTRACT. The Sandusky River basin, located in northwest Ohio, has been influenced by agriculture since the late-1800s. In 2003, the Ohio Environmental Protection Agency identified various tributaries of the Sandusky River as failing to meet biological water quality standards mainly due to siltation. To assess the effectiveness of best management practices (BMPs), a cutoff channel of the Sandusky River in Crawford County, Ohio was used as a unique archive of channel bed material that existed in the previous channel. Historical aerial photographs and USGS peak discharge data suggest the channel was likely abandoned between 1957 and 1964. Twelve sediment cores between 2 and 3 meters in depth were collected with a vibracore, and grain-size analyses of the cutoff channel substrate were compared to similar data collected from the modern channel. Results showed an historical fining-upward trend in the mean grain size of the coarse fraction, from gravel in the cutoff channel to sand in the modern channel, but no change in the mean grain size of the fine fraction. A series of alternative explanations were examined to elucidate this fining, including sediment storage, trends in population and crop cultivation, existence of BMPs, and sediment transport during floods. Evidence from this study strongly suggests that a shift from the cultivation of low-cover crops (hay and oats) to high-cover crops (corn and soybeans) has changed the proportion of coarse-grained to fine-grained sediment loading in this section of the Sandusky River. The results have implications both for the effectiveness of BMPs in Crawford County and possibly for Lake Erie sediment budgets. INDEX WORDS:
Sandusky River, channel substrate, vibracore, agricultural BMPs, Lake Erie.
channels (meandering, braided, anastomosed, and straight) (Miall 1992), meandering streams have unique properties, such as the preservation of channel substrate that occurs when meandering streams cutoff and abandon a meander. This property allows for the reconstruction of past flow regimes from cutoff channels (Allen 1965). Cutoff channels are often the end stage of a meander loop created by a combination of factors including: streams reaching a disorganized state and needing to self-organize, natural stream evolution, occurrence of high discharge events, and anthropogenic impacts (Hooke 2004). The two basic types of cutoff channels (chute and neck) allow for the reconstruction of past channel dimensions (Erskine et al. 1992). Moreover, substrate preserved in cutoffs can be compared to sub-
INTRODUCTION Fluvial deposits are among the most interesting and difficult features to study due to complexities associated with regional and local controls such as climate and tectonics. Anthropogenic impacts also play a role in how rivers behave. Sediment in runoff due to deforestation from logging, agriculture, or construction sites is among the most common of anthropogenic impacts. The combination of dynamic processes of a fluvial system with changes due to anthropogenic effects makes it difficult to analyze the extent of degradation in a river system because anthropogenic impacts mix with natural processes (Knox 2001). Of the four general types of fluvial *Corresponding
author. E-mail: [email protected]
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strate in modern channels because the base of a cutoff channel deposit consists of preserved substrates that are capped by infill sediment occurring from flood events and slumping of the banks. Schumm (1977) identified oxbows in the Murrumbidgee River basin, Australia, that had preserved the morphology of the stream from which he was able to determine the hydrological regime of the previous channel. Davidson et al. (2004) collected sediment cores from Sky Lake, an oxbow lake in Humphreys County, Mississippi and analyzed historic sediment accumulation rates and changes in erosion due to cultivation using isotopes. Mansuy et al. (2001) used sediment cores extracted from an oxbow lake near the Deûle River in the north of France to identify the nature and sources of pollutants. Piégay et al. (2002) investigated the evolution of eleven cutoff channels, particularly the silting-up dynamics, to determine their life span and potential for environmental preservation as wetlands. Best management practices, or BMPs, are guidelines created to balance water quality issues such as soil erosion with agricultural and urban development (OSU Extension 1991). The primary goal of agricultural BMPs is to decrease the impacts of agriculture on water quality by reducing the inputs of sediment from runoff and nutrients from fertilizers to streams (OSU Extension 1991). In Ohio, there are different recommended agricultural BMPs for land including forage, the conservation reserve program, no-till agriculture and mulch tillage. Agriculture has impacted the Sandusky River basin since the late-1800s. From the onset of agriculture in this region, there have been extensive construction of ditches and installation of underground tile drains to improve land drainage. Because of these impacts associated with agriculture, BMPs have been implemented in Crawford County, Ohio since 1987. The National Center for Water Quality Research (formerly Heidelberg Water Quality Laboratory) in Tiffin, Ohio has conducted research within the Sandusky River basin for several years. However, most of these studies have concentrated on non-point sources of suspended sediment or nutrients (e.g., Forster et al. 2000, Baker and Richards 2002, Richards and Grawbow 2003), not on channel substrate. The only previous studies completed in the Sandusky River basin to look at channel substrate and bedload materials used sediment cores to look at sediment accumulation in the Ballville reservoir for the purpose of modeling downstream sediment transport in the event of the removal of the Ballville
Dam (Evans et al. 2002, Evans and Gottgens 2007). The Sandusky River is one of the main tributaries to Lake Erie. Sandusky Bay, leading from the mouth of the Sandusky River to Lake Erie, is the most important fish nursery in the lake. Understanding the effects of the mostly agricultural watershed on the Sandusky River is of utmost importance for managing the water quality of Lake Erie and for the fate of the fishing industry in the lake. The objective of this study is to reconstruct preexisting channel substrate (channel bed materials) from a cutoff channel in order to analyze the extent of change in grain size with respect to the modern channel of the Sandusky River. This study answers two specific research questions: (1) Are vibracores collected from cutoff channels feasible for reconstructing pre-existing channel substrate? (2) How has channel substrate changed in this particular reach of the Sandusky River since the implementation of BMPs? METHODS Study Area The headwaters of the Sandusky River are located near the city of Crestline, Ohio. From its headwaters, the river flows mainly in a general westward direction until just south of the city of Upper Sandusky, where the river bends to flow in a northerly direction until it discharges into Sandusky Bay (Fig. 1). The Sandusky River is approximately 185 km (115 mi) long and has a drainage basin size of 3,680 km2 (1,421 mi2) (Flynn and Flynn 1904, Sherman 1932). Its flow is fragmented by four dams: the Ballville Dam, Bacon’s Dam, Ella Street Dam, and Indian Mill Dam, none of them upstream of the study site. Sherman (1932) described the topography of the basin as gently rolling except for areas near Upper Sandusky where the Sandusky River cuts through approximately 9 to 18 meters of glacial till. This glacial till is a mixture of predominantly unsorted clay containing small amounts of silt, sand, pebbles, and scattered boulders, and is a large source of the sediment for the basin. The bedrock, which is rather shallow, is the source for the majority of the material making up the glacial till that covers the landscape through which the Sandusky River flows (Forsyth 1975). The bedrock in the Sandusky River basin is composed of several units that are, from oldest to youngest: Silurian dolostone, Devonian limestone and shale, and Mississippian sandstone.
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Location of the Sandusky River basin and the study site.
Cutoff Channel Aerial photographs and topographic maps were used to identify potential cutoff channels in the Sandusky River basin. After several site visits, a suitable cutoff channel was located in Crawford County approximately 12 km (7.5 mi) east of the city of Bucyrus, Ohio (Fig. 1). The time of channel abandonment was approximated through the interpretation of historical topographic maps and aerial photographs coupled with peak streamflow data.
Historical aerial photographs obtained from the Crawford County Soil and Water Conservation District were available for 2 October 1939, 21 June 1957, 1 July 1964, 2 September 1970, and 19 September 1980. Annual peak streamflow data from 1922 to present were obtained from the closest United States Geological Survey (USGS) gauging station 04196500 at Sandusky River near Upper Sandusky, Ohio located 3.2 km (2 mi) northeast of the city of Upper Sandusky, and approximately 54.7 km (38 mi) downstream from the study site.
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FIG. 2. Location of collected samples. Vibracores are displayed as circles and modern channel substrates are displayed as triangles. Vibracores Initial field work at the study site consisted of seismic refraction profiling coupled with drilling to identify the depth to bedrock and to determine if fluvial deposits could be delineated using geophysical methods. A Geometrics SmartSeis 24 channel seismograph was used for the seismic work. Nine shot-to-depth (to determine depth-to-bedrock at sample locations) and two profiles (to determine bedrock scouring from the historic channel) were completed. In addition, a hydraulic drill rig was used to auger in the same locations as the seismic shots and profiles. Unfortunately, little information was gained using these methodologies. The fluvial deposits were too thin to be depicted using the seismic methods and augering did not allow for the precise identification of the channel substrate due to the mixing of the sediment. Therefore vibracores were collected to obtain suitable data for the study. Twelve vibracores were collected in July and August 2005 within the identified cutoff channel using
modified vibratory methods and equipment (e.g., Carter et al. 1982). The equipment for the collection of the vibracores consisted of a 5.5 m (15 ft) tripod, 7.6 cm (3.0 in) diameter seamless aluminum irrigation pipe (core), 3.5 h.p. engine, cable, and vibracore head. Recovered vibracores had lengths up to 4.5 m (15 ft) and displayed some compaction. The vibracores were split in the laboratory using a metal cutter and then photographed. Figure 2 shows the location of the vibracores obtained for this study. Vibracores VC-1, VC-2, VC3, VC-4, VC-5, VC-6, VC-10, VC-11, and VC-12 were collected within the cutoff channel to allow for the reconstruction of channel substrate that existed before channel abandonment. The reconstruction was performed by correlating the core depth and location of the identified channel substrate in the vibracores. Sediment core VC-9, located between the modern channel and the abandoned channel, was collected to represent the oldest preserved sediment that would have been deposited during
Effectiveness of Best Management Practices lateral accretion of the channel. Sediment cores VC-7 and VC-8 were collected to document the furthest extent of lateral-accretion of the previous channel. Cutoff channel substrate was identified in the vibracores using subsurface facies analysis. Cutoff channel substrate contained gravel, sand, silt, and clay with incorporated shell fragments or whole shells. In addition, the channel substrate identified in the vibracores was capped with litter debris consisting of twigs and leaves, which typically caps the channel substrate layer. After the cutoff channel substrate was identified, it was sampled for grainsize analysis. Grain-size analysis for the cutoff channel substrate was completed using standard sieving techniques for sand (e.g., Gale and Hoare 1991) and a Malvern Mastersizer 2000 laser particle analyzer was used for silt and clay. One particular problem was recognizing the base of the channel substrate layer if the underlying material was glacial till. To accomplish this, detailed grain-size analyses were conducted of the fine-grained material underlying the cutoff channel substrate from vibracores VC-1, VC-3, VC-5, and VC-7 due to the different appearance of the fine-grained material when moist, and to be certain that the underlying material was not associated with the cutoff channel substrate. Modern Channel Four substrate samples were collected in March 2006 in the modern channel to allow for a comparison of cutoff channel substrate and modern channel substrate. Modern channel substrate was sampled using a 7.6 cm (3 in) diameter PVC pipe, which was manually pushed into the channel substrate and capped. The modern substrate was sampled with this methodology to be comparable to the data collected from the vibracores, which were collected with 7.6 cm (3 in) diameter aluminum irrigation pipe. The push cores were split open in the laboratory and their grain size analyzed using the same methods as in the vibracores. RESULTS Cutoff Channel Observations of the different stages of channel abandonment from historical aerial photographs suggest that the Sandusky River abandoned the meander loop between 21 June 1957 and 1 July 1964. As shown in Figure 3C, the channel had not yet
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abandoned the meander loop on 21 June 1957. However, by 1 July 1964 the channel had abandoned the meander loop and was active in the present location, while the cutoff channel was in the oxbow/channel fill stage (Fig. 3D). By 2 September 1970, the cutoff channel was filled and vegetated (Fig. 3E). Therefore, from the historical aerial photographs, the distinct channel shifting occurred between 21 June 1957 and 1 July 1964. To refine the approximate time of channel abandonment, annual peak flow data from the closest USGS gauging station was used. The closest USGS gauging station is station 04196500 at Sandusky River near Upper Sandusky, Ohio, located 3.2 km (2 mi) northeast of the city of Upper Sandusky and approximately 54.7 km (38 mi) downstream from the study site. Analysis of the USGS annual peak streamflow data shows three large discharge events occurring on 22 January 1959 with 283.2 m 3/s (10,000 cfs), 6 March 1963 with 192.6 m 3 /s (6,800 cfs), and 22 April 1964 with 217.2 m 3 /s (7,670 cfs) (Fig. 4). It is likely that the event of channel cutoff occurred during the largest peak flood event on 22 January 1959. Vibracores The correlation diagram of the cutoff channel substrate from six (VC-1, VC-4, VC-5, VC-10, VC11, and VC-12) of the nine vibracores collected within the cutoff channel is shown in Figure 5. Of the rest of the vibracores collected in the cutoff channel, VC-2 and VC-3 were not correlated due to the lack of identifiable channel substrate, and no correlation with VC-6 was possible because the channel substrate layer was not penetrated. Additional vibracores were collected outside the cutoff channel (VC-7, VC-8, and VC-9). Vibracores VC-7 and VC-8 presented soil on top of glacial till with no lateral accretion surfaces or channel substrate suggesting the channel did not laterally accrete any further east of the cutoff channel. VC-9 displayed a lateral accretion surface, however, data from this core represent much older sediment, probably pre-1915, the date of the earliest record that shows the channel already in the cutoff location (Fig. 3A), and thus cannot be correlated with other vibracores collected within the cutoff channel. Grain-size analysis performed on the fine-grained material below the cutoff channel substrate from vibracores VC-1, VC-3, VC-5, and VC-7 showed that the fine-grained underlying material was poorly sorted with an estimated graphic mean greater than
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FIG. 3. Cutoff channel abandonment. Historical topographic map from 1915 (A) and historical aerial photographs (B), (C), (D), and (E).
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FIG. 4. Annual peak stream flow data from USGS gauging station 04196500 at Sandusky River near Upper Sandusky (the period of channel abandonment is outlined). 4Φ (Φ = – log2 D), which is consistent with glacial till from northwest Ohio (Forsyth 1975). Grain-size analysis was also carried out on the correlated channel substrates identified in the cutoff from vibracores VC-1, VC-4, VC-5, VC-10, VC-11, and VC-12, and on VC-3 and on VC-9. VC-3 contained very fine-grained material thought to be channel substrate. However, further inspection of VC-3 showed no shell fragments or capping litter debris to identify a substrate layer. Figure 6 shows the percent gravel, sand, silt and clay for the vibracores in the cutoff with identifiable channel substrate, plus VC-9, and samples from the modern channel. The six vibracores from the cutoff channel have a low silt-clay content that varied from 6.4% in VC-1 to 37.5% in VC-4, with the average being 15.0%. As expected in a meandering channel, the silt-clay content was lowest at the inflection point (or crossing) and highest from pools near the apex of the bend. The six vibracores from the cutoff channel had high gravel contents, averaging 55.8% (ranging from 45.1% gravel in VC-4 to 73.1% in VC-5), while the sand content averaged 28.9% (ranging from 17.1% sand in VC-4 and VC-5 to 39.4% in VC-10). VC-3 seems to be an outlier. The percentage of silt and clay for sediment sampled in VC-3 at the depth other channel substrates were identified in the correlated cutoff channel substrates were much
higher (81.7%). It also had very low percentages of gravel and somewhat low percentages of sand with respect to the other vibracores. VC-3 is interpreted to represent the outer cut-bank of the channel. Figures 7 and 8 present the graphic mean and the inclusive standard deviation for the vibracores and modern channel samples. Figure 9 shows the graphic mean and inclusive standard deviation plotted against each other to illustrate spatial trends in sorting and mean grain size. The average inclusive standard deviation value of 2.4Φ from the correlated channel substrates indicates that they were very poorly sorted (Folk and Ward 1957). Modern Channel Four modern channel samples were collected in March 2006 and analyzed using the same grain-size methodologies as the cutoff channel. Figure 6 shows the percent gravel, sand, silt, and clay for the modern channel samples. Of the four samples, C-2 displayed the highest percentage of silt and clay with 32.2%. The average silt and clay content from the modern channel substrate was 12.4% (ranging from 4.3% in C-3 to 32.2% in C-2), while the average gravel content was 45.3% (ranging from 30.3% in C-3 to 64.7% in C-4), and the average sand content was 41.6% (ranging from 29.4% in C-4 to
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FIG. 5. Correlation diagram of the cutoff channel substrate from the vibracores collected in July and August 2005.
65.3% in C-3, which indicates a possible submerged sandbar). Figures 7, 8, and 9 present the graphic mean and the inclusive standard deviation for the modern channel samples. The average inclusive standard deviation of 2.25Φ for the modern channel samples indicates that the substrate is very poorly sorted (Folk and Ward 1957).
DISCUSSION Cutoff Channel Channels often abandon meander loops during large discharge events (Erskine et al. 1992). USGS peak streamflow data from gauging station 04196500 at Sandusky River near Upper Sandusky, Ohio showed three large discharge events occurring
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FIG. 6. Percent gravel, sand, and silt and clay for the cutoff channel substrate (VC-#) and modern channel substrate (C-#). during the period of 21 June 1957 and 1 July 1964. It is likely that the meander loop was abandoned during the largest of these discharges on 22 January 1959, dating the channel substrate preserved in the cutoff to approximately 47 years of age. The probable sequence of events was chute reactivation and deepening during a flood, followed by abandonment of the old meander bend channel, and subsequent deposition of fine-grained flood deposits into the old channel. It is probable that the older flood was responsible because it was the largest event and because this would have provided sufficient time for the cutoff channel to be filled and vegetated prior to the aerial photograph of 2 September 1970 (Fig. 3E). The correlation diagram in Figure 5 displays three to four layers of sand that overlie the pre-cutoff channel substrates in VC-1, VC-10, and VC-12. These sand layers are interpreted as postcutoff flood events where sediments from the main channel overtopped into the abandoned channel. Similarly, VC-4 and VC-11 show one layer of sand and VC-5 show two layers of sand overlying the pre-cutoff channel substrate. It is possible that the additional flood events were not recorded in VC-4, VC-5, and VC-11 due to bank failure and slumping, which could have occurred prior to the 6 March 1963 and 22 April 1964 high discharge events.
Pre-cutoff Channel Substrates The six vibracores collected in the cutoff display great potential for reconstructing the pre-cutoff channel substrate. Subsurface facies analysis of the channel substrate from these six vibracores shows that they all contain varying amounts of gravel, sand, silt, and clay as well as incorporated leaf litter, whole shells, and shell fragments. The stratigraphy and grain-size characteristics exhibited in each core conform to the spatial distribution of the cores around a meander bend (Fig. 5). For example, the lack of lateral accretion deposits in cores VC-7 and VC-8 demonstrates that the channel had not migrated any further east. Similarly, VC-3, collected near the apex of the cutoff, was initially thought to contain a very fine-grained channel substrate. However, a closer look at this core showed small amounts of particulate organic matter and no shell fragments, suggesting that these sediments were not channel substrate. Furthermore, grain-size data from VC-3 appears as an outlier when plotted (Figs. 7 to 9). VC-3 was likely the outer cut-bank of the channel. VC-2 also lacks a channel substrate layer, and is interpreted as the outer cut-bank, similar to VC-3. VC-9 was collected between the modern channel location and the cutoff channel. Subsurface facies analysis from VC-9 displays a lateral accretion sur-
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FIG. 7. Graphic mean of the coarse fraction from the grainsize analysis of cutoff channel substrate, modern channel substrate, and glacial till (note the fining of substrate from the cutoff channel substrate to modern channel substrate). face. The sediment in this core would have been deposited when the channel migrated laterally eastward. The oldest available documentation, a 1915 historical topographic map (Fig. 3A), indicates that the channel was already further eastward than the location of VC-9, suggesting that the sediments in this core are pre-1915. However, sediment identified in VC-9 should not be considered unaltered pre-settlement substrate. By 1915 agriculture had already been in the region for at least 55 years and the exact time the channel was at the location of VC-9 is unknown. Contrast of Pre- and Post-cutoff Substrates Fine-grained sediment in both the cutoff channel and modern channel show similar spatial trends. For example, there are lesser amounts of silt and clay near the inflection points (crossings) and greater amounts near the bend apex interpreted as the deposition in pools (Fig. 6). Temporally, we expected to find a lower percentage of silt and clay in the modern channel substrate versus the cutoff channel as a result of the implementation of BMPs since 1987. This expected decrease of the fine fraction material did not appear in the data collected.
In contrast, the coarse-grained fraction shows a significant difference between the substrate in the pre-cutoff channel and the modern channel. The coarse fraction from the cutoff channel had an average inclusive standard deviation of 2.43Φ and an average mean grain size of –1.52Φ (Fig. 9), while the modern channel substrate had an average inclusive standard deviation of 2.25Φ and an average mean grain size of –0.92Φ (Fig. 9). This difference is significant; because it means the mean grain size of the pre-cutoff channel was gravel while the mean grain size of the modern channel is sand. Interestingly, there is no significant change in sorting between the cutoff channel and the modern channel; both channels are poorly sorted (Folk and Ward 1957). The historical change from gravel substrates to sand substrates was not expected because the primary goal of BMPs is to decrease the impacts of agriculture on water quality by reducing the finegrained sediment inputs. Causes of Historical Changes in Substrate There are several possible explanations for the fining of the channel substrate from the cutoff channel around the late 1950s to the modern channel.
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FIG. 8. Inclusive standard deviation for cutoff channel substrate, modern channel substrate, and glacial till.
FIG. 9. Inclusive standard deviation vs. graphic mean for cutoff channel deposits, modern channel substrate, and glacial till.
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These explanations are based both on the development of the cutoff and on the changes in agricultural practices that have taken place around the study area. Explanation 1: Cutoff Channels Preserve Coarser Substrate Cutoff channels typically occur during large discharge events when the channel is transporting coarse-grained sediment. Thus it is possible that the cutoff preserves coarser-grained sediment being transported during a large discharge event that was not representative of normal flow conditions. However, meander bends are complex environments that would be expected to show a diversity of grain sizes, modified by subsequent flow events. Also, once a meander loop is abandoned, it initially takes the form of an oxbow lake where suspended sediments will settle out on top of previous channel substrate, and infiltrate into the coarse-grained substrate. Therefore, it is more likely that cutoff channel substrates would result in an overestimate of fine-grained sediment from processes associated with the infilling of the cutoff channel, rather than the reverse. Explanation 2: Lack of Data from BMPs Implemented Upstream It has been difficult to reconstruct the specific locations where BMPs have been implemented in Crawford County. It is possible that BMPs have not been implemented immediately upstream from the study site. In general, stream habitat improvements linked to implementation of BMPs (such as coarsening of channel substrate) only occur in the locations where stream buffers and bank stabilization have been in place for some period of time and only if the watershed is in reasonably good health (Wang et al. 2002). In this case, the area around the study site is used for pasture, and the area upstream has a mature riparian corridor, with vegetation at least 10–15 years old. In other words, if anything one would expect the modern channel to show substrate improvements, rather than the reverse. Explanation 3: Sediment Storage Although the modern channel substrate is finergrained than the pre-cutoff channel substrate, it is still possible that sedimentation loadings have decreased in Crawford County due to implementation of BMPs in 1987. Numerous studies have docu-
mented increased stream sedimentation loadings in response to mining, deforestation, and agriculture and also the reduction of sediment loadings as a consequence of implementing better land management practices such as crop rotations and contour plowing (Knox 1987, Trimble 1999, Fitzpatrick and Knox 2000, Steegen et al. 2001, Evans et al. 2002). This study could not independently document historical patterns in stream sediment loadings, but data were available from a study of the Ballville Reservoir downstream (Evans et al. 2002). Historically, high mass sedimentation rates occurred in the Ballville Reservoir during 1927–1933 (4.3 g/cm 2 /yr), 1950–1959 (2.9 g/cm 2 /yr), and 1978–1993 (2.7 g/cm2/yr), in association with large flood events. In contrast, the mass sedimentation rate during 1960–1978 was 1.7 g/cm2/yr (Evans et al. 2002). These data suggest no significant change in stream sediment loadings when comparing 1950–1959 (approximately the pre-cutoff channel) with 1978–1993 (approximately the modern channel). However, the two sites are sufficiently far apart that local variability might be masked. Explanation 4: Effect of Land-use Changes Changes in human population, size of cattle herds, or changes in crop selection in Crawford County may have caused the fining of channel substrate observed in this study. The first two suggestions can be dismissed. Although the human population has increased by an order of magnitude (from 4,791 people in the 1830s to 46,966 in 2002), with a sharp increase between 1940–1970, most of this growth was downstream of the study site (Pruitt 1998). In addition, cattle herd size in Crawford County has declined since these records were first available in 1975. Cattle declined from 24,000 head of cattle in 1975 to 9,900 head of cattle in 2005. Although cattle trampling on stream banks can cause increased stream loadings, it is unlikely that the decreasing number of cattle during the study interval would have caused the fining of the channel substrate documented in this study. The percentage of land in agriculture in Crawford County has stayed relatively constant, at around 90% farmland, since the first known census in 1860. However, there is a distinct trend in the type of crops harvested per acre from 1918 to 2005. The number of acres of corn and soybeans harvested has increased since the 1950s, while the amount of acres of harvested hay and oats has been reduced considerably. This trend shows an increase in acres
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FIG. 10. Total harvested high cover crops and low cover crops for Crawford County, OH.
of high-cover crops planted in rows like corn and soybeans and a reduction in areas of low-cover crops not planted in row patterns (Fig. 10). Crops planted in rows such as high-cover crops leave distinct paths for water to flow, and also leave the ground bare for longer periods of time compared to low-cover crops, increasing the potential for runoff and soil erosion (Ministry of Agriculture 1987). This shift from low-cover crops to high-cover crops most likely increased the sediment load in the waterways of Crawford County and caused the fining of the modern channel substrate with respect to the cutoff channel. CONCLUSION A cutoff channel located in Crawford County, 12 km (7.5 mi) east of Bucyrus, Ohio, was used to determine the effectiveness of agricultural BMPs in the upper section of the Sandusky River basin. This river basin has been influenced by agriculture since the late-1800s and has failed to meet biological water quality standards mainly due to siltation. Agricultural BMPs have been implemented on this section of the basin since 1987. This study showed that vibracores collected from a cutoff channel allow the reconstruction of a pre-
existing channel substrate and, therefore, a relative comparison with the modern channel substrate. Other studies have determined sedimentation rates for this and other basins, but have not investigated changes to the channel substrate. Species distribution of fish and macroinvertebrates are related to channel substrate. The location of physical aquatic habitat is partly determined by the channel substrate size. Common carp (an introduced species) is found where degradation has produced good habitat for them in the form of fine-grained substrate. The grain-size analyses from this study determined that the pre-cutoff channel had a gravel (–1.52Φ) substrate while the modern channel has a sand (–0.92Φ) substrate. This reduction in mean grain size (0.6Φ or ~1 mm) in substrate is thought to be the result of changes in agricultural practices from planting low-cover crops (oats and hay) to high-cover crops (corn and soybeans). Given the documented fining of channel substrate from this study, it is suggested that changes in agricultural practices may have offset or negated improvements from implementations of BMPs. This result was not anticipated, and additional work should be done to document spatial and temporal changes in sedimentation rates in the upper Sandusky River basin. With the proliferation of watershed coalitions and
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river action programs in the Lake Erie watershed, it is important to realize that the simple application of agricultural BMPs without regard to the type of crop planted may reduce the effectiveness of BMPs. These results may have implications on the water quality, sediment loads, and aquatic habitats of tributary streams for Lake Erie and the other Great Lakes. ACKNOWLEDGMENTS We would like to thank the Ohio Department of Natural Resources, Division of Geological Survey, Lake Erie Geology Group for lending the vibracoring equipment and materials, and the property owner of the study site for granting permission to complete this project. REFERENCES Allen, J.R.L. 1965. A review of the origin and characteristics of recent alluvial sediments. Sedimentology 5:89–191. Baker, D.B., and Richards, R.P. 2002. Relationships between changing phosphorus budgets and riverine phosphorus export in northwestern Ohio watersheds. Journal of Environmental Quality 31:96–108. Carter, C.H., Williams, S.J., Fuller, J.A., and Meisburger, E.P. 1982. Regional geology of the southern Lake Erie (Ohio) bottom: A seismic reflection and vibracore study. Coastal Engineering Research Center, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. Miscellaneous Report No 82–15. Davidson, G.R., Carnley, M., Lange, T., Galicki, S.J., and Douglas, A. 2004. Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agriculture use: A Pb-210, Cs-137, and C-14 study in Mississippi, USA. Radiocarbon 46(2):755–764. Erskine, W., McFadden, C., and Bishop, P. 1992. Alluvial cutoffs as indicators of former channel conditions. Earth Surface Process and Landforms 17: 23–37. Evans, J.E., and Gottgens, J.F. 2007. Contaminant stratigraphy of the Ballville Reservoir, Sandusky River, NW Ohio: implications for dam removal. J. Great Lakes Res. 33 (Special Issue 2):182–193. ——— , Levine, N.S., Roberts, S.J., Gottgens, J.F., and Newman, D.M. 2002. Assessment using GIS and sediment routing of the proposed removal of Ballville Dam, Sandusky River, Ohio. Journal of the American Water Resources Association 38(6):1549–1565. Fitzpatrick, F.A., and Knox, J.C. 2000. Spatial and temporal sensitivity of hydrogeomorphic response and
recovery to deforestation, agriculture, and floods. Physical Geography 21(2):89–108. Folk, R.L., and Ward, W.C. 1957. Brazos River bar: A study in the significance of the grain size parameters. Journal of Sedimentary Petrology 27:3–26. Forster, D., Lynn, R., Richards, R.P., Baker, D.B., and Blue, E.N. 2000. Relating water quality to farming practices: A case study from the Lake Erie Basin. Journal of Soil and Water Conservation 55:85–90. Forsyth, J.L. 1975. The geological setting of the Sandusky River Basin. In Proceedings, Sandusky River Basin Symposium, D.B. Baker, W.B. Jackson, and B.L. Prater, eds., pp. 13–60. International Joint Commission. Flynn, B.H., and Flynn, M.S. 1904. The natural features and economic development of the Sandusky, Maumee, Muskingum, and Miami drainage areas in Ohio. United States Geological Survey. Water Supply and Irrigation Paper No 91. Gale, S.J., and Hoare, P.G. 1991. Quaternary Sediments: Petrographic Methods for the Study of Unlithified Rocks. New York, NY: John Wiley and Sons. Hooke, J.M. 2004. Cutoffs galore!: Occurrence and causes of multiple cutoffs on a meandering river. Geomorphology 61:225–238. Knox, J.C. 1987. Historical valley floor sedimentation in the upper Mississippi Valley. Annals of the Association of American Geographers 77 (2):224–244. ——— . 2001. Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley. Catena 42:193–224. Mansuy, L., Bourezgui, Y., Garnier-Zarli, E., Jardé, E., and Réveillé, V. 2001. Characterization of humic substances in highly polluted river sediments by pyroloysis methylation-gas chromatography-mass spectrometry. Organic Geochemistry 32:223–231. Miall, A.D. 1992. Alluvial Deposits. In Facies Models, Response to sea level change, R.G. Walker and N.P. James, eds., pp. 119–142. Geological Association of Canada, St. Johns, Newfoundland. Ministry of Agriculture. 1987. Food and Rural Affairs, Fact Sheet, Soil Erosion—Causes and Effects. Queens Printer for Ontario, 572, 87-040. Ohio State University Extension. 1991. Agricultural Best Management Practices. Fact Sheet AEX-464-91. Piégay, H., Bornette, G., and Grante, P. 2002. Assessment of silting-up dynamics of eleven cut-off channel plugs on a free-meandering river (Ain River, France). In Applied Geomorphology: Theory and Practice, R.J. Allison, ed., chap. 14, pp. 227–247. New York, NY: John Wiley and Sons. Pruitt, B.H. 1998. Timken: From Missouri to Mars—A Century of Leadership in Manufacturing. Cambridge, MA: Harvard Business School Press. Richards, R.P., and Grabow, G.L. 2003. Detecting reductions in sediment loads associated with Ohio’s Con-
Effectiveness of Best Management Practices servation Reserve Enhancement Program. Journal of the American Water Resources Association 39(5):1261–1268. Schumm, S.A. 1977. The Fluvial System. New York, NY: John Wiley and Sons. Sherman, C.E. 1932. Ohio Stream Flow, Part 1, Areas of lakes and drainage basins run-off records prior to 1921. Ohio State University Studies, The Engineering Experiment Station Bulletin No. 73, Vol. 1, No. 5. Steegen, A., Govers, G., Takken, I., Nachtergaele, J., Poesen, J., and Merckx, R. 2001. Landscape and watershed processes, factors controlling sediment and phosphorus export from two Belgian agricultural
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catchments. Journal of Environmental Quality 30:1249–1258. Trimble, S.W. 1999. Decreased rates of alluvial sediment storage in the Coon Creek Basin, Wisconsin, 1975–1993. Science 285(5431):1244–1246. Wang, L., Lyons, J., and Kanehl, P. 2002. Effects of watershed best management practices on habitat and fish in Wisconsin streams. Journal of the American Water Resources Association 38(3):663–680. Submitted: 6 November 2006 Accepted: 4 May 2007 Editorial handling: James E. Evans
Using Pre-existing Channel Substrate to Determine the Effectiveness of Best Management Practices, Sandusky River, Ohio Ryan P. Murphy1, Enrique Gomezdelcampo2,*, and James E. Evans2 1Hull
& Associates, Inc. 3401 Glendale Avenue, Suite 300 Toledo, Ohio 43614 2School
of Earth, Environment and Society Bowling Green State University Bowling Green, Ohio 43403-0211
ABSTRACT. The Sandusky River basin, located in northwest Ohio, has been influenced by agriculture since the late-1800s. In 2003, the Ohio Environmental Protection Agency identified various tributaries of the Sandusky River as failing to meet biological water quality standards mainly due to siltation. To assess the effectiveness of best management practices (BMPs), a cutoff channel of the Sandusky River in Crawford County, Ohio was used as a unique archive of channel bed material that existed in the previous channel. Historical aerial photographs and USGS peak discharge data suggest the channel was likely abandoned between 1957 and 1964. Twelve sediment cores between 2 and 3 meters in depth were collected with a vibracore, and grain-size analyses of the cutoff channel substrate were compared to similar data collected from the modern channel. Results showed an historical fining-upward trend in the mean grain size of the coarse fraction, from gravel in the cutoff channel to sand in the modern channel, but no change in the mean grain size of the fine fraction. A series of alternative explanations were examined to elucidate this fining, including sediment storage, trends in population and crop cultivation, existence of BMPs, and sediment transport during floods. Evidence from this study strongly suggests that a shift from the cultivation of low-cover crops (hay and oats) to high-cover crops (corn and soybeans) has changed the proportion of coarse-grained to fine-grained sediment loading in this section of the Sandusky River. The results have implications both for the effectiveness of BMPs in Crawford County and possibly for Lake Erie sediment budgets. INDEX WORDS:
Sandusky River, channel substrate, vibracore, agricultural BMPs, Lake Erie.
channels (meandering, braided, anastomosed, and straight) (Miall 1992), meandering streams have unique properties, such as the preservation of channel substrate that occurs when meandering streams cutoff and abandon a meander. This property allows for the reconstruction of past flow regimes from cutoff channels (Allen 1965). Cutoff channels are often the end stage of a meander loop created by a combination of factors including: streams reaching a disorganized state and needing to self-organize, natural stream evolution, occurrence of high discharge events, and anthropogenic impacts (Hooke 2004). The two basic types of cutoff channels (chute and neck) allow for the reconstruction of past channel dimensions (Erskine et al. 1992). Moreover, substrate preserved in cutoffs can be compared to sub-
INTRODUCTION Fluvial deposits are among the most interesting and difficult features to study due to complexities associated with regional and local controls such as climate and tectonics. Anthropogenic impacts also play a role in how rivers behave. Sediment in runoff due to deforestation from logging, agriculture, or construction sites is among the most common of anthropogenic impacts. The combination of dynamic processes of a fluvial system with changes due to anthropogenic effects makes it difficult to analyze the extent of degradation in a river system because anthropogenic impacts mix with natural processes (Knox 2001). Of the four general types of fluvial *Corresponding
author. E-mail: [email protected]
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strate in modern channels because the base of a cutoff channel deposit consists of preserved substrates that are capped by infill sediment occurring from flood events and slumping of the banks. Schumm (1977) identified oxbows in the Murrumbidgee River basin, Australia, that had preserved the morphology of the stream from which he was able to determine the hydrological regime of the previous channel. Davidson et al. (2004) collected sediment cores from Sky Lake, an oxbow lake in Humphreys County, Mississippi and analyzed historic sediment accumulation rates and changes in erosion due to cultivation using isotopes. Mansuy et al. (2001) used sediment cores extracted from an oxbow lake near the Deûle River in the north of France to identify the nature and sources of pollutants. Piégay et al. (2002) investigated the evolution of eleven cutoff channels, particularly the silting-up dynamics, to determine their life span and potential for environmental preservation as wetlands. Best management practices, or BMPs, are guidelines created to balance water quality issues such as soil erosion with agricultural and urban development (OSU Extension 1991). The primary goal of agricultural BMPs is to decrease the impacts of agriculture on water quality by reducing the inputs of sediment from runoff and nutrients from fertilizers to streams (OSU Extension 1991). In Ohio, there are different recommended agricultural BMPs for land including forage, the conservation reserve program, no-till agriculture and mulch tillage. Agriculture has impacted the Sandusky River basin since the late-1800s. From the onset of agriculture in this region, there have been extensive construction of ditches and installation of underground tile drains to improve land drainage. Because of these impacts associated with agriculture, BMPs have been implemented in Crawford County, Ohio since 1987. The National Center for Water Quality Research (formerly Heidelberg Water Quality Laboratory) in Tiffin, Ohio has conducted research within the Sandusky River basin for several years. However, most of these studies have concentrated on non-point sources of suspended sediment or nutrients (e.g., Forster et al. 2000, Baker and Richards 2002, Richards and Grawbow 2003), not on channel substrate. The only previous studies completed in the Sandusky River basin to look at channel substrate and bedload materials used sediment cores to look at sediment accumulation in the Ballville reservoir for the purpose of modeling downstream sediment transport in the event of the removal of the Ballville
Dam (Evans et al. 2002, Evans and Gottgens 2007). The Sandusky River is one of the main tributaries to Lake Erie. Sandusky Bay, leading from the mouth of the Sandusky River to Lake Erie, is the most important fish nursery in the lake. Understanding the effects of the mostly agricultural watershed on the Sandusky River is of utmost importance for managing the water quality of Lake Erie and for the fate of the fishing industry in the lake. The objective of this study is to reconstruct preexisting channel substrate (channel bed materials) from a cutoff channel in order to analyze the extent of change in grain size with respect to the modern channel of the Sandusky River. This study answers two specific research questions: (1) Are vibracores collected from cutoff channels feasible for reconstructing pre-existing channel substrate? (2) How has channel substrate changed in this particular reach of the Sandusky River since the implementation of BMPs? METHODS Study Area The headwaters of the Sandusky River are located near the city of Crestline, Ohio. From its headwaters, the river flows mainly in a general westward direction until just south of the city of Upper Sandusky, where the river bends to flow in a northerly direction until it discharges into Sandusky Bay (Fig. 1). The Sandusky River is approximately 185 km (115 mi) long and has a drainage basin size of 3,680 km2 (1,421 mi2) (Flynn and Flynn 1904, Sherman 1932). Its flow is fragmented by four dams: the Ballville Dam, Bacon’s Dam, Ella Street Dam, and Indian Mill Dam, none of them upstream of the study site. Sherman (1932) described the topography of the basin as gently rolling except for areas near Upper Sandusky where the Sandusky River cuts through approximately 9 to 18 meters of glacial till. This glacial till is a mixture of predominantly unsorted clay containing small amounts of silt, sand, pebbles, and scattered boulders, and is a large source of the sediment for the basin. The bedrock, which is rather shallow, is the source for the majority of the material making up the glacial till that covers the landscape through which the Sandusky River flows (Forsyth 1975). The bedrock in the Sandusky River basin is composed of several units that are, from oldest to youngest: Silurian dolostone, Devonian limestone and shale, and Mississippian sandstone.
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FIG. 1.
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Location of the Sandusky River basin and the study site.
Cutoff Channel Aerial photographs and topographic maps were used to identify potential cutoff channels in the Sandusky River basin. After several site visits, a suitable cutoff channel was located in Crawford County approximately 12 km (7.5 mi) east of the city of Bucyrus, Ohio (Fig. 1). The time of channel abandonment was approximated through the interpretation of historical topographic maps and aerial photographs coupled with peak streamflow data.
Historical aerial photographs obtained from the Crawford County Soil and Water Conservation District were available for 2 October 1939, 21 June 1957, 1 July 1964, 2 September 1970, and 19 September 1980. Annual peak streamflow data from 1922 to present were obtained from the closest United States Geological Survey (USGS) gauging station 04196500 at Sandusky River near Upper Sandusky, Ohio located 3.2 km (2 mi) northeast of the city of Upper Sandusky, and approximately 54.7 km (38 mi) downstream from the study site.
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FIG. 2. Location of collected samples. Vibracores are displayed as circles and modern channel substrates are displayed as triangles. Vibracores Initial field work at the study site consisted of seismic refraction profiling coupled with drilling to identify the depth to bedrock and to determine if fluvial deposits could be delineated using geophysical methods. A Geometrics SmartSeis 24 channel seismograph was used for the seismic work. Nine shot-to-depth (to determine depth-to-bedrock at sample locations) and two profiles (to determine bedrock scouring from the historic channel) were completed. In addition, a hydraulic drill rig was used to auger in the same locations as the seismic shots and profiles. Unfortunately, little information was gained using these methodologies. The fluvial deposits were too thin to be depicted using the seismic methods and augering did not allow for the precise identification of the channel substrate due to the mixing of the sediment. Therefore vibracores were collected to obtain suitable data for the study. Twelve vibracores were collected in July and August 2005 within the identified cutoff channel using
modified vibratory methods and equipment (e.g., Carter et al. 1982). The equipment for the collection of the vibracores consisted of a 5.5 m (15 ft) tripod, 7.6 cm (3.0 in) diameter seamless aluminum irrigation pipe (core), 3.5 h.p. engine, cable, and vibracore head. Recovered vibracores had lengths up to 4.5 m (15 ft) and displayed some compaction. The vibracores were split in the laboratory using a metal cutter and then photographed. Figure 2 shows the location of the vibracores obtained for this study. Vibracores VC-1, VC-2, VC3, VC-4, VC-5, VC-6, VC-10, VC-11, and VC-12 were collected within the cutoff channel to allow for the reconstruction of channel substrate that existed before channel abandonment. The reconstruction was performed by correlating the core depth and location of the identified channel substrate in the vibracores. Sediment core VC-9, located between the modern channel and the abandoned channel, was collected to represent the oldest preserved sediment that would have been deposited during
Effectiveness of Best Management Practices lateral accretion of the channel. Sediment cores VC-7 and VC-8 were collected to document the furthest extent of lateral-accretion of the previous channel. Cutoff channel substrate was identified in the vibracores using subsurface facies analysis. Cutoff channel substrate contained gravel, sand, silt, and clay with incorporated shell fragments or whole shells. In addition, the channel substrate identified in the vibracores was capped with litter debris consisting of twigs and leaves, which typically caps the channel substrate layer. After the cutoff channel substrate was identified, it was sampled for grainsize analysis. Grain-size analysis for the cutoff channel substrate was completed using standard sieving techniques for sand (e.g., Gale and Hoare 1991) and a Malvern Mastersizer 2000 laser particle analyzer was used for silt and clay. One particular problem was recognizing the base of the channel substrate layer if the underlying material was glacial till. To accomplish this, detailed grain-size analyses were conducted of the fine-grained material underlying the cutoff channel substrate from vibracores VC-1, VC-3, VC-5, and VC-7 due to the different appearance of the fine-grained material when moist, and to be certain that the underlying material was not associated with the cutoff channel substrate. Modern Channel Four substrate samples were collected in March 2006 in the modern channel to allow for a comparison of cutoff channel substrate and modern channel substrate. Modern channel substrate was sampled using a 7.6 cm (3 in) diameter PVC pipe, which was manually pushed into the channel substrate and capped. The modern substrate was sampled with this methodology to be comparable to the data collected from the vibracores, which were collected with 7.6 cm (3 in) diameter aluminum irrigation pipe. The push cores were split open in the laboratory and their grain size analyzed using the same methods as in the vibracores. RESULTS Cutoff Channel Observations of the different stages of channel abandonment from historical aerial photographs suggest that the Sandusky River abandoned the meander loop between 21 June 1957 and 1 July 1964. As shown in Figure 3C, the channel had not yet
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abandoned the meander loop on 21 June 1957. However, by 1 July 1964 the channel had abandoned the meander loop and was active in the present location, while the cutoff channel was in the oxbow/channel fill stage (Fig. 3D). By 2 September 1970, the cutoff channel was filled and vegetated (Fig. 3E). Therefore, from the historical aerial photographs, the distinct channel shifting occurred between 21 June 1957 and 1 July 1964. To refine the approximate time of channel abandonment, annual peak flow data from the closest USGS gauging station was used. The closest USGS gauging station is station 04196500 at Sandusky River near Upper Sandusky, Ohio, located 3.2 km (2 mi) northeast of the city of Upper Sandusky and approximately 54.7 km (38 mi) downstream from the study site. Analysis of the USGS annual peak streamflow data shows three large discharge events occurring on 22 January 1959 with 283.2 m 3/s (10,000 cfs), 6 March 1963 with 192.6 m 3 /s (6,800 cfs), and 22 April 1964 with 217.2 m 3 /s (7,670 cfs) (Fig. 4). It is likely that the event of channel cutoff occurred during the largest peak flood event on 22 January 1959. Vibracores The correlation diagram of the cutoff channel substrate from six (VC-1, VC-4, VC-5, VC-10, VC11, and VC-12) of the nine vibracores collected within the cutoff channel is shown in Figure 5. Of the rest of the vibracores collected in the cutoff channel, VC-2 and VC-3 were not correlated due to the lack of identifiable channel substrate, and no correlation with VC-6 was possible because the channel substrate layer was not penetrated. Additional vibracores were collected outside the cutoff channel (VC-7, VC-8, and VC-9). Vibracores VC-7 and VC-8 presented soil on top of glacial till with no lateral accretion surfaces or channel substrate suggesting the channel did not laterally accrete any further east of the cutoff channel. VC-9 displayed a lateral accretion surface, however, data from this core represent much older sediment, probably pre-1915, the date of the earliest record that shows the channel already in the cutoff location (Fig. 3A), and thus cannot be correlated with other vibracores collected within the cutoff channel. Grain-size analysis performed on the fine-grained material below the cutoff channel substrate from vibracores VC-1, VC-3, VC-5, and VC-7 showed that the fine-grained underlying material was poorly sorted with an estimated graphic mean greater than
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FIG. 3. Cutoff channel abandonment. Historical topographic map from 1915 (A) and historical aerial photographs (B), (C), (D), and (E).
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FIG. 4. Annual peak stream flow data from USGS gauging station 04196500 at Sandusky River near Upper Sandusky (the period of channel abandonment is outlined). 4Φ (Φ = – log2 D), which is consistent with glacial till from northwest Ohio (Forsyth 1975). Grain-size analysis was also carried out on the correlated channel substrates identified in the cutoff from vibracores VC-1, VC-4, VC-5, VC-10, VC-11, and VC-12, and on VC-3 and on VC-9. VC-3 contained very fine-grained material thought to be channel substrate. However, further inspection of VC-3 showed no shell fragments or capping litter debris to identify a substrate layer. Figure 6 shows the percent gravel, sand, silt and clay for the vibracores in the cutoff with identifiable channel substrate, plus VC-9, and samples from the modern channel. The six vibracores from the cutoff channel have a low silt-clay content that varied from 6.4% in VC-1 to 37.5% in VC-4, with the average being 15.0%. As expected in a meandering channel, the silt-clay content was lowest at the inflection point (or crossing) and highest from pools near the apex of the bend. The six vibracores from the cutoff channel had high gravel contents, averaging 55.8% (ranging from 45.1% gravel in VC-4 to 73.1% in VC-5), while the sand content averaged 28.9% (ranging from 17.1% sand in VC-4 and VC-5 to 39.4% in VC-10). VC-3 seems to be an outlier. The percentage of silt and clay for sediment sampled in VC-3 at the depth other channel substrates were identified in the correlated cutoff channel substrates were much
higher (81.7%). It also had very low percentages of gravel and somewhat low percentages of sand with respect to the other vibracores. VC-3 is interpreted to represent the outer cut-bank of the channel. Figures 7 and 8 present the graphic mean and the inclusive standard deviation for the vibracores and modern channel samples. Figure 9 shows the graphic mean and inclusive standard deviation plotted against each other to illustrate spatial trends in sorting and mean grain size. The average inclusive standard deviation value of 2.4Φ from the correlated channel substrates indicates that they were very poorly sorted (Folk and Ward 1957). Modern Channel Four modern channel samples were collected in March 2006 and analyzed using the same grain-size methodologies as the cutoff channel. Figure 6 shows the percent gravel, sand, silt, and clay for the modern channel samples. Of the four samples, C-2 displayed the highest percentage of silt and clay with 32.2%. The average silt and clay content from the modern channel substrate was 12.4% (ranging from 4.3% in C-3 to 32.2% in C-2), while the average gravel content was 45.3% (ranging from 30.3% in C-3 to 64.7% in C-4), and the average sand content was 41.6% (ranging from 29.4% in C-4 to
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FIG. 5. Correlation diagram of the cutoff channel substrate from the vibracores collected in July and August 2005.
65.3% in C-3, which indicates a possible submerged sandbar). Figures 7, 8, and 9 present the graphic mean and the inclusive standard deviation for the modern channel samples. The average inclusive standard deviation of 2.25Φ for the modern channel samples indicates that the substrate is very poorly sorted (Folk and Ward 1957).
DISCUSSION Cutoff Channel Channels often abandon meander loops during large discharge events (Erskine et al. 1992). USGS peak streamflow data from gauging station 04196500 at Sandusky River near Upper Sandusky, Ohio showed three large discharge events occurring
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FIG. 6. Percent gravel, sand, and silt and clay for the cutoff channel substrate (VC-#) and modern channel substrate (C-#). during the period of 21 June 1957 and 1 July 1964. It is likely that the meander loop was abandoned during the largest of these discharges on 22 January 1959, dating the channel substrate preserved in the cutoff to approximately 47 years of age. The probable sequence of events was chute reactivation and deepening during a flood, followed by abandonment of the old meander bend channel, and subsequent deposition of fine-grained flood deposits into the old channel. It is probable that the older flood was responsible because it was the largest event and because this would have provided sufficient time for the cutoff channel to be filled and vegetated prior to the aerial photograph of 2 September 1970 (Fig. 3E). The correlation diagram in Figure 5 displays three to four layers of sand that overlie the pre-cutoff channel substrates in VC-1, VC-10, and VC-12. These sand layers are interpreted as postcutoff flood events where sediments from the main channel overtopped into the abandoned channel. Similarly, VC-4 and VC-11 show one layer of sand and VC-5 show two layers of sand overlying the pre-cutoff channel substrate. It is possible that the additional flood events were not recorded in VC-4, VC-5, and VC-11 due to bank failure and slumping, which could have occurred prior to the 6 March 1963 and 22 April 1964 high discharge events.
Pre-cutoff Channel Substrates The six vibracores collected in the cutoff display great potential for reconstructing the pre-cutoff channel substrate. Subsurface facies analysis of the channel substrate from these six vibracores shows that they all contain varying amounts of gravel, sand, silt, and clay as well as incorporated leaf litter, whole shells, and shell fragments. The stratigraphy and grain-size characteristics exhibited in each core conform to the spatial distribution of the cores around a meander bend (Fig. 5). For example, the lack of lateral accretion deposits in cores VC-7 and VC-8 demonstrates that the channel had not migrated any further east. Similarly, VC-3, collected near the apex of the cutoff, was initially thought to contain a very fine-grained channel substrate. However, a closer look at this core showed small amounts of particulate organic matter and no shell fragments, suggesting that these sediments were not channel substrate. Furthermore, grain-size data from VC-3 appears as an outlier when plotted (Figs. 7 to 9). VC-3 was likely the outer cut-bank of the channel. VC-2 also lacks a channel substrate layer, and is interpreted as the outer cut-bank, similar to VC-3. VC-9 was collected between the modern channel location and the cutoff channel. Subsurface facies analysis from VC-9 displays a lateral accretion sur-
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FIG. 7. Graphic mean of the coarse fraction from the grainsize analysis of cutoff channel substrate, modern channel substrate, and glacial till (note the fining of substrate from the cutoff channel substrate to modern channel substrate). face. The sediment in this core would have been deposited when the channel migrated laterally eastward. The oldest available documentation, a 1915 historical topographic map (Fig. 3A), indicates that the channel was already further eastward than the location of VC-9, suggesting that the sediments in this core are pre-1915. However, sediment identified in VC-9 should not be considered unaltered pre-settlement substrate. By 1915 agriculture had already been in the region for at least 55 years and the exact time the channel was at the location of VC-9 is unknown. Contrast of Pre- and Post-cutoff Substrates Fine-grained sediment in both the cutoff channel and modern channel show similar spatial trends. For example, there are lesser amounts of silt and clay near the inflection points (crossings) and greater amounts near the bend apex interpreted as the deposition in pools (Fig. 6). Temporally, we expected to find a lower percentage of silt and clay in the modern channel substrate versus the cutoff channel as a result of the implementation of BMPs since 1987. This expected decrease of the fine fraction material did not appear in the data collected.
In contrast, the coarse-grained fraction shows a significant difference between the substrate in the pre-cutoff channel and the modern channel. The coarse fraction from the cutoff channel had an average inclusive standard deviation of 2.43Φ and an average mean grain size of –1.52Φ (Fig. 9), while the modern channel substrate had an average inclusive standard deviation of 2.25Φ and an average mean grain size of –0.92Φ (Fig. 9). This difference is significant; because it means the mean grain size of the pre-cutoff channel was gravel while the mean grain size of the modern channel is sand. Interestingly, there is no significant change in sorting between the cutoff channel and the modern channel; both channels are poorly sorted (Folk and Ward 1957). The historical change from gravel substrates to sand substrates was not expected because the primary goal of BMPs is to decrease the impacts of agriculture on water quality by reducing the finegrained sediment inputs. Causes of Historical Changes in Substrate There are several possible explanations for the fining of the channel substrate from the cutoff channel around the late 1950s to the modern channel.
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FIG. 8. Inclusive standard deviation for cutoff channel substrate, modern channel substrate, and glacial till.
FIG. 9. Inclusive standard deviation vs. graphic mean for cutoff channel deposits, modern channel substrate, and glacial till.
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These explanations are based both on the development of the cutoff and on the changes in agricultural practices that have taken place around the study area. Explanation 1: Cutoff Channels Preserve Coarser Substrate Cutoff channels typically occur during large discharge events when the channel is transporting coarse-grained sediment. Thus it is possible that the cutoff preserves coarser-grained sediment being transported during a large discharge event that was not representative of normal flow conditions. However, meander bends are complex environments that would be expected to show a diversity of grain sizes, modified by subsequent flow events. Also, once a meander loop is abandoned, it initially takes the form of an oxbow lake where suspended sediments will settle out on top of previous channel substrate, and infiltrate into the coarse-grained substrate. Therefore, it is more likely that cutoff channel substrates would result in an overestimate of fine-grained sediment from processes associated with the infilling of the cutoff channel, rather than the reverse. Explanation 2: Lack of Data from BMPs Implemented Upstream It has been difficult to reconstruct the specific locations where BMPs have been implemented in Crawford County. It is possible that BMPs have not been implemented immediately upstream from the study site. In general, stream habitat improvements linked to implementation of BMPs (such as coarsening of channel substrate) only occur in the locations where stream buffers and bank stabilization have been in place for some period of time and only if the watershed is in reasonably good health (Wang et al. 2002). In this case, the area around the study site is used for pasture, and the area upstream has a mature riparian corridor, with vegetation at least 10–15 years old. In other words, if anything one would expect the modern channel to show substrate improvements, rather than the reverse. Explanation 3: Sediment Storage Although the modern channel substrate is finergrained than the pre-cutoff channel substrate, it is still possible that sedimentation loadings have decreased in Crawford County due to implementation of BMPs in 1987. Numerous studies have docu-
mented increased stream sedimentation loadings in response to mining, deforestation, and agriculture and also the reduction of sediment loadings as a consequence of implementing better land management practices such as crop rotations and contour plowing (Knox 1987, Trimble 1999, Fitzpatrick and Knox 2000, Steegen et al. 2001, Evans et al. 2002). This study could not independently document historical patterns in stream sediment loadings, but data were available from a study of the Ballville Reservoir downstream (Evans et al. 2002). Historically, high mass sedimentation rates occurred in the Ballville Reservoir during 1927–1933 (4.3 g/cm 2 /yr), 1950–1959 (2.9 g/cm 2 /yr), and 1978–1993 (2.7 g/cm2/yr), in association with large flood events. In contrast, the mass sedimentation rate during 1960–1978 was 1.7 g/cm2/yr (Evans et al. 2002). These data suggest no significant change in stream sediment loadings when comparing 1950–1959 (approximately the pre-cutoff channel) with 1978–1993 (approximately the modern channel). However, the two sites are sufficiently far apart that local variability might be masked. Explanation 4: Effect of Land-use Changes Changes in human population, size of cattle herds, or changes in crop selection in Crawford County may have caused the fining of channel substrate observed in this study. The first two suggestions can be dismissed. Although the human population has increased by an order of magnitude (from 4,791 people in the 1830s to 46,966 in 2002), with a sharp increase between 1940–1970, most of this growth was downstream of the study site (Pruitt 1998). In addition, cattle herd size in Crawford County has declined since these records were first available in 1975. Cattle declined from 24,000 head of cattle in 1975 to 9,900 head of cattle in 2005. Although cattle trampling on stream banks can cause increased stream loadings, it is unlikely that the decreasing number of cattle during the study interval would have caused the fining of the channel substrate documented in this study. The percentage of land in agriculture in Crawford County has stayed relatively constant, at around 90% farmland, since the first known census in 1860. However, there is a distinct trend in the type of crops harvested per acre from 1918 to 2005. The number of acres of corn and soybeans harvested has increased since the 1950s, while the amount of acres of harvested hay and oats has been reduced considerably. This trend shows an increase in acres
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FIG. 10. Total harvested high cover crops and low cover crops for Crawford County, OH.
of high-cover crops planted in rows like corn and soybeans and a reduction in areas of low-cover crops not planted in row patterns (Fig. 10). Crops planted in rows such as high-cover crops leave distinct paths for water to flow, and also leave the ground bare for longer periods of time compared to low-cover crops, increasing the potential for runoff and soil erosion (Ministry of Agriculture 1987). This shift from low-cover crops to high-cover crops most likely increased the sediment load in the waterways of Crawford County and caused the fining of the modern channel substrate with respect to the cutoff channel. CONCLUSION A cutoff channel located in Crawford County, 12 km (7.5 mi) east of Bucyrus, Ohio, was used to determine the effectiveness of agricultural BMPs in the upper section of the Sandusky River basin. This river basin has been influenced by agriculture since the late-1800s and has failed to meet biological water quality standards mainly due to siltation. Agricultural BMPs have been implemented on this section of the basin since 1987. This study showed that vibracores collected from a cutoff channel allow the reconstruction of a pre-
existing channel substrate and, therefore, a relative comparison with the modern channel substrate. Other studies have determined sedimentation rates for this and other basins, but have not investigated changes to the channel substrate. Species distribution of fish and macroinvertebrates are related to channel substrate. The location of physical aquatic habitat is partly determined by the channel substrate size. Common carp (an introduced species) is found where degradation has produced good habitat for them in the form of fine-grained substrate. The grain-size analyses from this study determined that the pre-cutoff channel had a gravel (–1.52Φ) substrate while the modern channel has a sand (–0.92Φ) substrate. This reduction in mean grain size (0.6Φ or ~1 mm) in substrate is thought to be the result of changes in agricultural practices from planting low-cover crops (oats and hay) to high-cover crops (corn and soybeans). Given the documented fining of channel substrate from this study, it is suggested that changes in agricultural practices may have offset or negated improvements from implementations of BMPs. This result was not anticipated, and additional work should be done to document spatial and temporal changes in sedimentation rates in the upper Sandusky River basin. With the proliferation of watershed coalitions and
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river action programs in the Lake Erie watershed, it is important to realize that the simple application of agricultural BMPs without regard to the type of crop planted may reduce the effectiveness of BMPs. These results may have implications on the water quality, sediment loads, and aquatic habitats of tributary streams for Lake Erie and the other Great Lakes. ACKNOWLEDGMENTS We would like to thank the Ohio Department of Natural Resources, Division of Geological Survey, Lake Erie Geology Group for lending the vibracoring equipment and materials, and the property owner of the study site for granting permission to complete this project. REFERENCES Allen, J.R.L. 1965. A review of the origin and characteristics of recent alluvial sediments. Sedimentology 5:89–191. Baker, D.B., and Richards, R.P. 2002. Relationships between changing phosphorus budgets and riverine phosphorus export in northwestern Ohio watersheds. Journal of Environmental Quality 31:96–108. Carter, C.H., Williams, S.J., Fuller, J.A., and Meisburger, E.P. 1982. Regional geology of the southern Lake Erie (Ohio) bottom: A seismic reflection and vibracore study. Coastal Engineering Research Center, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. Miscellaneous Report No 82–15. Davidson, G.R., Carnley, M., Lange, T., Galicki, S.J., and Douglas, A. 2004. Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agriculture use: A Pb-210, Cs-137, and C-14 study in Mississippi, USA. Radiocarbon 46(2):755–764. Erskine, W., McFadden, C., and Bishop, P. 1992. Alluvial cutoffs as indicators of former channel conditions. Earth Surface Process and Landforms 17: 23–37. Evans, J.E., and Gottgens, J.F. 2007. Contaminant stratigraphy of the Ballville Reservoir, Sandusky River, NW Ohio: implications for dam removal. J. Great Lakes Res. 33 (Special Issue 2):182–193. ——— , Levine, N.S., Roberts, S.J., Gottgens, J.F., and Newman, D.M. 2002. Assessment using GIS and sediment routing of the proposed removal of Ballville Dam, Sandusky River, Ohio. Journal of the American Water Resources Association 38(6):1549–1565. Fitzpatrick, F.A., and Knox, J.C. 2000. Spatial and temporal sensitivity of hydrogeomorphic response and
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catchments. Journal of Environmental Quality 30:1249–1258. Trimble, S.W. 1999. Decreased rates of alluvial sediment storage in the Coon Creek Basin, Wisconsin, 1975–1993. Science 285(5431):1244–1246. Wang, L., Lyons, J., and Kanehl, P. 2002. Effects of watershed best management practices on habitat and fish in Wisconsin streams. Journal of the American Water Resources Association 38(3):663–680. Submitted: 6 November 2006 Accepted: 4 May 2007 Editorial handling: James E. Evans