Soil creep in a mesotidal salt marsh channel bank: Fast, seasonal, and water table mediated

Abstract

Muddy banks of marsh channels experience soil creep – a viscous-like slow deformation resulting in a net downslope transport. Here we present the first field evidence of soil creep in a mesotidal salt marsh using both pole displacement and high precision measurements of soil deformation taken with a vibrating-wire extensometer over two years. Soil extended 2–4 mm/m/year in the high marsh platform and 20–40 mm/m/year in the low marsh bank. This was equivalent to a soil diffusivity of about 0.2 m2/year, which is much higher than the diffusivity on hillslopes and also matches the high value predicted by long-term marsh evolution models. High precision (±0.03 mm) soil deformation measurements shed light on the dynamics of the creep process. First, the low bank compressed and extended by ~5 mm/m every tidal cycle, suggesting that the diurnal changes in water level play a role in the bank movements. Second, net soil extension took place mostly during fall. Net soil extension preceded the period of ice formation, suggesting that freezing and ice rafting are not a leading cause of soil creep. Net soil extension was likely triggered by an increase in water table during fall, which decreased the effective stress and thus destabilized the bank. This high water table was caused by a combination of an abiotic factor – the increase in atmospheric precipitation – and a biotic factor – the decrease in evapotranspiration triggered by vegetation senescence. This study confirms that creep is a significant process in marsh morphodynamics and that vegetation can reduce mass wasting in marsh channels.