Sedimentation Rate Change in the Winooski River Delta

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Trevor Kennedy
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1 Sedimentation Rate Change in the Winooski River Delta Chris Ricker and Brian Connelly Abstract Historical air photographs, from 1937 show the Winooski River Delta extended much farther into Lake Champlain that modern photos and surveying show the delta today. Two major changes took place to the river since The River narrowed significantly since 1974 (old woman) and more significantly to deltaic processes, reforestation has reduced the amount of sediment flux in to the lake. When river water reaches Lake Champlain it slows significantly, loosing energy and depositing river sediment in the delta. The change in erosion rate was determined by comparing the 1937 delta to the modern delta. A change in volume was calculated by using basic trigonometric and geometric functions (old air photo, new air photo, cross-sectional area). The change in volume was calculated to be 1.0*10^8 cubic meters. Further calculations can then show the change in sedimentation rates.

2 3. The image (page 5) is a 1937 air photo taken for soil conservation purposes. It covers several km in each direction and is centered near the Ethan Allen Homestead. It covers much of the lower Winooski River. A large delta can be observed, complete with sandbars well offshore. The Delta is enhanced in post processing for clarity. There are many farm fields but few forests. An oxbow lake is visible north of the river. Two bridges, one, near the lake and one more inland, cross the river. Although most of the 50% of the land that was deforested lies in relative proximity to the river, much of the increased sediment flux originated from upland regions. There is significant marshland to each side of the river near the delta, where water has spread into low-lying areas. Riparian Vegetation is nearly non-existent, present along only three stretches; one to the lower right of the picture, one outside the large bend, and one on the delta. Human development greatly influenced the river and delta. Deforestation can be seen as a leading cause for the expanded delta. There are many large fields and smaller parcels, sometimes interspersed with trees. This represents an increase in tree cover over older reports. Soft, wet ground has probably prevented the building and farming too near the delta. 4. Have rates of sediment deposition in the Winooski River Delta increased or decreased as a function of Vermont s changing land use practices, and by how much? 6. Location Easting Northing Burlington bridge Burlington delta Colchester bridge Colchester delta Table 1: GPS coordinates of Winooski River Delta

3 8. Cross-sectional area: 2.3m * 80m = 184.0m 2 Discharge: 0.529m/s * 184 m 2 = 97.4 m 3 /s (it should be noted that discharge was observed during flooding). Channel type: alluvial river delta into large lake. Soils: 2 horizons were distinguishable under exposed roots on the south bank of the Winooski River. A small dark O layer, 1-3 cm thick, was composed of organic matter (twigs, roots, leaves) at varying stages of decay. Below was the A horizon, up to 20cm thick (limited by minimal exposure along bank.). This sandy layer was unsorted and graded with finer grain closer to the top of the strata. 9. Due to high water during surveying on Oct. 19 th some data is estimative. Depth could only be found at one location and the research group carried only three oranges, preventing detailed surface velocity measurements. Discharge was found to be 97.4 cubic meters per second based on a depth of 2.3m * a width of 80m * an average velocity of.529 m/s. Manning's n was found to be.123. n= [R^(2/3) * S^(1/2)]/ Velocity. R was found to be 159.6/ Slope was determined from a topographic map. The distance between the lake and the 100 foot contour was 1km. The average lake level was found to be 95 feet. < 5 feet = m, so the slope is 1.524/1000 = m/m. 10. Despite continued inputs of sediment into the Winooski River Delta the size of the delta since 1937, the size of the delta has been reduced by 1.0 * 10 8 cubic metes. Sedimentation rates, and therefore erosion, were39x higher than today, averaged over the Winooski Drainage Basin. Although hill slope instability could be observed along the river, it is in the uplands where hill slope stability affects the Winooski Delta the most. While deforested, the uplands faced more erosion and landslides, increasing the sediment load in the Winooski. When reforested, the hill slopes became more stable and decreased the sediment flux into the lake. An interesting follow-up to this study would be an in depth evaluation of sediment retention in the delta. Has the amount of sediment flowing out of the system changed with time as rates of deposition have?

4 11. During the height of the Lauentide Ice sheet, the Winooski Delta area was completely covered by ice. As the ice retreated Glacial Lake Vermont formed, with its shores at high elevations. Further retreat caused sea water to enter into the Champlain Basin forming the Champlain Sea. Soon the ground rebounded and caused the formation of Lake Champlain where the Winooski began building the delta. 12. Deforestation for pasture and active agriculture in the early 20 th century left most of Vermont s hillsides with little or no structures to keep soil in place. This loss of soil cohesion resulted in drastically higher rates of erosion and landslides. The gradual reforestation of the Vermont landscape has resulted in a significantly reduced amount of sediment reaching the Winooski River Delta. 13. Dissolved load: ICP Data: Al Be Ca Fe K Mg Na Ti Total mg/l DisolvedLoad: DischargeconvertedtoLiters per second 97* , 000 L = s Dischargexconcentration: L mg 2.16 kg * = s L s At recorded discharge levels, 2.16kg of dissolved load flows through the river every second. The Discharge was converted to Liters per second from cubic meters. The discharge was multiplied by the total amount, of mg per liter.

5 References Charlie Auld. Verbal interview. October 19, Nice older lady; name unknown. October 24, =3

6 Modern Winooski River Delta Charlie Auld s Boathouse 70m Bike Path Bridge Delta outlined and shaded. Chris Ricker and Brian Connelly Water Sample Location Old Womans Hhouse From Google.com

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