Influence of Floodplain Subsurface Composition on Peatland Stream Channel Mobility

Abstract Peatlands are important for the earth's carbon cycle, globally storing the equivalent of two‐thirds of the atmospheric carbon reservoir. The capacity of peatlands to preserve organic matter is influenced by stream‐channel mobility and sediment reworking; however, the nature of these processes is poorly constrained. This study investigates influences on peatland stream migration at Cedar Creek (Minnesota, USA). Aerial imagery, sediment coring, flow monitoring, radiocarbon dating, and an analytical bank‐erosion model were used to assess the morphodynamic processes of Cedar Creek. The bank‐erosion model was calibrated using empirical effective‐erodibility coefficients derived from an aerial‐imagery assessment of lateral mobility. Two study sections were identified: an upstream segment dominated (>95%) by peat, and a downstream segment containing a mixture of clastic (sandy) sediment and peat. Morphological aspects of the channel coincide with this compositional change, including a transition from angular (“zigzag”) bends to more rounded meanders with point‐bar deposits composed of sand. Importantly, migration rates vary significantly: channel mobility in the mixed clastic‐peat segment is approximately twice that of the peat‐dominated segment. The difference coincides with the abundance of sand found in the channel, bankline, and floodplain deposits. Bankline sand is not delivered via overbank flooding; instead, this material has been incorporated into channel‐floodplain deposits via stream migration since the inception of Cedar Creek with deglaciation, as clastic and organic material aggraded the channel‐peatland system simultaneously. The cohesive property of peat is the primary inhibitor of channel migration. This study presents a framework for assessing how peat‐stream mobility may respond to future hydrological changes.