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  1. 6 5 4 3 1 2 N 1 mile Organic deposits associated with an avulsion belt, forming in Cumberland Marshes of Saskatchewan, Canada Thomas Tran and K. Siân Davies-Vollum IAS Program in Environmental Sciences, University of Washington, Tacoma Introduction The Cumberland Marshes are located in the east central part of Saskatchewan in Canada (Fig. 1). These marshes span an area of 5000 km2 (Smith et al. 1989). Fluvial features in this region include floodplains (Fig. 2), levees, marshes, wetlands, lakes, bogs, old and new channels (Fig. 3), and crevasse splays. The marshes were formed due to the avulsion of the Saskatchewan River (Morozova and Smith 2000). An avulsion results from the changing pathway of a river from established flow into a new course, usually at a lower altitude (Smith et al. 1989, Slingerland and Smith 2004). Before the 1870’s, the Saskatchewan River consisted of a single channel. After that time, the river was diverted northward due to an ice jam. This resulted in the avulsion belt (Smith et al. 1989). This belt now consists of many small channels and is an unstable area due to the frequent channel switching and sediment deposition (Morozova and Smith 2000). Organic-rich materials are known to be deposited on river floodplains, such as those found near the Saskatchewan River (Morozova and Smith 2002). Organic substrates need a stable setting to form without sediment-input (Morozova and Smith 2002). Consequently, layers of organic materials continue to accrete in the absence of inorganic debris. The accumulation rate of organic deposits is dependent on two factors: sedimentation rates and the length of isolation. The lower the sedimentation rate and the longer the length of the floodplain isolation, the higher the total organic carbon (TOC) content (Morozova and Smith 2002). Methodology Samples were taken using a gouge auger (Fig. 4) from six different floodplains inside and outside the avulsion belt: North Angling meander, Cadotte Complex, Steamboat floodplain, North Mossy Loop, West Mossy floodplain, and East Mossy floodplain (Fig. 1). The samples were taken 50 meters apart along transects. The core depth varied depending on how much sediments were recovered (Fig. 5). The top sediment samples (0-15 cm) were taken by the hand-grab technique because the top sediment samples usually fell off when the samples were pulled up. The TOC content was determined for all samples using the loss-on-ignition method (Andrejko et al. 1983, Dean 1974). The samples were air-dried and 3-6 grams of each sample was used for analysis. These samples were then analyzed and compared between sites. The samples were then placed into the muffle furnace at 100 oC for 2 hours. This allowed evaporation of all water from the samples. After the samples were cooled, they were weighed again to get the dry sample weight. Finally, they were placed back into the muffle furnace at 500 oC for 5 hours to burn off the organic content. The total organic content is then calculated by finding the difference between the final weight and dry weight and calculating this as a percentage of the dry weight. The results were then graphed using scatter plots with the distance across floodplain on the x-axis and the average and surface % TOC content on the y-axis. Results Results are shown below for the TOC content of surface substrate (Fig. 6-11) and core average (Fig. 12-17) for six floodplain transects. • Discussion • Floodplain trends in organic content: Results indicate that floodplain deposits with greatest TOC content are found farther away from the active river channels both inside and outside the avulsion belt (Figs. 6-11, Figs. 13-17) except surface TOC content for the North Angling floodplain (Fig. 12). The average organic content of cores and surface organic content increases with distance away from the levee. The R2 value greater than 0.75 indicates there is a strong correlation. The average TOC content of the cores and the surface substrate for the North Mossy loop floodplain increases from the levee to a distance of 200 meters and then decreases from that distance to the other side of the loop because this transect crosses the floodplain from one channel to another (Fig. 11, Fig. 17). • 2) Average organic content of cores: Organic deposits are usually formed in stable areas. These are often located outside of the avulsion belt because the organic matter accumulates where floodplains have been isolated, so the sediment deposition onto the floodplain is limited (Morozova and Smith 2002).The West Mossy floodplain has the highest average TOC content and it does not lie in the avulsion belt. This floodplain has the greatest amount of peat because it has been more isolated from in-flow of sedimentation through flooding (Dirschl 1972) and thus has been stable. Other floodplains have lower average TOC content and more heterogeneous floodplain substrate. This is because they lie inside the avulsion belt and are more unstable with more frequent influxes of sediments. These results agree with a study from Morozova and Smith (2000). • 3) Surface organic content: The results show that surficial organic-rich deposits inside and outside the avulsion belt have relatively equal amounts of TOC content. The surface TOC content for the West Mossy floodplain, which lies just outside the avulsion belt, is approximately equal with the surface TOC content from many other floodplains that are inside the avulsion belt. The results illustrate that the organic matter does deposit inside and outside the avulsion belt and not only in stable areas. More floodplains outside the avulsion belt could be studied to confirm this. Cumberland House Figure 6-11: Graphs of surface % TOC content across six floodplain transects from levee Conclusions Our study of floodplains associated with an avulsion belt in the Cumberland Marshes of Saskatchewan, indicates that organic deposits are currently forming inside and outside the avulsion belt. This implies that organic materials can accumulate under the unstable conditions of river avulsion. Our results also show that the accumulation of organic materials increases across floodplains with distance from active river channels in all locations, regardless of whether the floodplain is within or outside the avulsion belt. Map source: Washington University archives. N 3 miles Figure 2: A meander loop floodplain Figure 3: Old and new channels Cumberland House Literature cited Andrejko, M.J., F. Fiene, and A.D. Cohen. 1983. Comparison of ashing techniques for determination of the inorganic content of peats. Testing of Peats and Organic Soils. ASTM STP 820, P.M. Jarrett, Ed., American Society for Testing and Materials. pp. 15-20. Dean, W.E.Jr. 1974. Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: Comparison with other methods. Journal of Sedimentary Petrology. 44(1): 242-248. Dirschl, H.J. 1972. Geobotanical Processes in the Saskatchewan River Delta. Canadian Journal of Earth Science 9(11): 1529-1549. Morozova, G. and N.D. Smith. 2002. Organic matter deposition in the Saskatchewan River floodplain (Cumberland Marshes, Canada): effects of progradational avulsions. Sedimentary Geology 3074: 1-15. Morozova, G. and N.D. Smith. 2000. Holocene avulsion styles and sedimentation patterns of the Saskatchewan River, Cumberland Marshes, Canada. Sedimentary Geology 130: 81-105. Slingerland, R. and N.D. Smith. 2004. River avulsions and their deposits. Annual Review Earth Planetary Science 32: 257-285. Smith, N.D., T.A. Cross, J.P. Dufficy, and S.R. Clough. 1989. Anatomy of an avulsion. Sedimentology 36: 1-23. Courtesy of Norm D. Smith • Figure 1: Map of North America and Landsat images of Cumberland Marshes, Saskatchewan, Canada. The map shows the location of the Cumberland Marshes in eastern Saskatchewan. The numbers on the projected Landsat image represent six different sampled floodplains. • The North Angling floodplain: inside southern of avulsion belt • The Cadotte Complex floodplain: inside southern of avulsion belt • The Steamboat floodplain: inside western of avulsion belt • The East Mossy loop: inside northern of avulsion belt • The West Mossy loop: outside northern of avulsion belt • The North Mossy loop: inside northern of avulsion belt Figure 4: Samples in the field are taken using a gouge auger Figure 5: Samples were divided up at different lengths Acknowledgments We would like to thank a local guide, Gary Carriere, and Tim Dillavou, and Norm D. Smith for field assistance. Also, I would like to thank Jeannie Friel for assisting me on this project. Figure 12-17: Graphs of average % TOC content across six floodplain transects from levee For further information Please contact vtrann@u.washington.edu & ksdavies@u.washington.edu