Influence of Aquatic Vegetation on Channel Hydraulics, Morphology, and Seasonal Accretion in Tidal Freshwater Marsh Inlet Channels

Abstract:

We examined interactions among aquatic vegetation, flow hydraulics, sediment (organic and inorganic) deposition, organic matter decomposition, and the channel form of tidal freshwater marsh inlet channels. Inlets chosen for study were partially covered by a dominate vegetation species (N.luteum, Z.aquatica, or H.verticullata). Vegetation cover, height, stem diameter, and stem density were measured monthly along each channel cross section. Water surface elevation was measured with multiple gauges in each tidal inlet and accompanied by measurements of velocity. These data were used to calculate total and effective channel shear stresses. Channel cross section elevations were surveyed bimonthly to determine elevation change in inlets occupied by each of the three dominate vegetation species. Sediment cores were obtained along each inlet cross section and analyzed for bulk density, grain size, and percentage of organic matter. Leaf litter experiments were conducted to determine plant decomposition rates. The three aquatic plants grew in significantly different water depths (Z. aquatica the shallowest and H. verticullata the deepest). Z. aquatica and N. luteum had similar stem diameters and densities and grew over well-defined platforms associated with low shear stresses during vegetation maxima. The central channel core, however, had higher summer shear stresses than predicted from depth and slope data. The zones of low shear stress persisted after vegetation die-back, but shear stresses in the central core decreased during non-vegetated periods. The deep central cores of these seasonally-vegetated inlets experienced erosion during the warm season and deposition during cool (unvegetated) periods. The H. verticullata channels had a parabolic channel form rather than the platform-central form observed in the other channels. Decomposition experiments indicated significantly higher decomposition rates for H. verticullata and N. luteum than for Z. aquatica. Comparison among tidal inlets indicated an inverse relationship between maximum inlet depth and average channel accretion rate. These results suggest that initial vegetation colonization modifies channel inlet morphology and that both vegetation and morphology generate the shear stress distributions that maintain channel form.