Characterizing channel migration habits, sediment-transport capacity and flow dynamics in peatlands: an example from Cedar Creek, Minnesota U.S.A.
Peatlands are essential to the global carbon cycle as the terrestrial carbon storage capacity of these environments may store the equivalent of two-thirds, if not more, of atmospheric carbon. Of particular importance to the carbon flux and storage capacity of a peatland are the hydrologic conditions and morphodynamics of fluvial channels as they directly impact the evolution of these environments, primarily through migration and partitioning of channelized surface flow. However, peatland channel mechanics and morphodynamics are poorly understood. This thesis investigates the properties that influence migration rate of channels in peat-dominated landscapes by analyzing flow characteristics, substrate composition, and migration habits of Cedar Creek in central Minnesota and the peatland through which it flows. Two main reaches of the creek were selected as the focus of this study, one upstream of a valley constriction and another downstream of a constriction point in the valley. Analysis of sediment cores, numerical modeling of the sediment transport conditions and bank erosion, and analysis of aerial imagery, were employed to quantify the spatial and temporal variability of channel mobility observed in Cedar Creek. Core analysis shows markedly different substrate compositions between the two reaches. In the downstream reach there are extensive sand bodies intermixed with peat soil. The upstream reach is dominated by peat with little to no sand in the subsurface. Field based estimates of the migration rate range from a minimum of 4.30 mm/yr to a maximum 11.21 mm/yr (timescale of migration of 490-1280 yrs.) in the downstream reach. Numerical models of erosion conducted by applying alluvial models of bank erosion estimate an erosion rate of 9.04 mm/yr in the downstream reach. In the upstream it is not possible to estimate a migration rate without channel deposits in the subsurface. However, numerical modeling of the erosion rate in the upstream reach is estimated as 0.42 mm/yr (timescale of migration of ~13100 yrs.). The spatial variability of channel mobility is primarily attributed to the prevalence of sand in the subsurface. It is likely what controls mobility of peatland channels and can shift the system from a peat-dominated to alluvial-process-dominated system over centuries to millennia as a peatland is reworked by changing hydrological processes that destabilize peatland growth and development of peat soils.
Peatlands, Geomorphology, Sedimentology, Fluvial, Alluvial, Carbon, Erosion, Channel Mobility
Networked Digital Library of Theses and Dissertations