Floating rooted hydrophytes and flow and sediment dynamics: Field study in secondary channels of the Paraná River

Abstract

Worldwide, aquatic vegetation is a key component in many river systems due to its diverse interactions as well as the environmental services it provides. In the Paraná fluvial system, large mats of floating rooted hydrophytes are present in channels with different water conditions whose interaction with flow and sediment dynamics is currently unknown. This work aimed to evaluate the temporal coverage variation of floating rooted hydrophytic mats as well as the alteration of flow dynamic and sediment deposition associated with their presence in natural conditions. Four different secondary channels were analyzed for two different and consecutive water stages: mean and high-water levels. Hydrophytic coverage ranged from 5% to 67% and from about 3% to 14% of the total discharge drained throughout the vegetated areas of the channels, being the lowest values recorded during the high-water stage. The specific discharge distribution and flow velocities at the vegetated cross-sections were altered when compared to non-vegetated sections. Mean velocities and depths were markedly lower inside vegetation mats. Additionally, fine sediments (silt and clay) dominated the bed samples below the hydrophytic mats of the vegetated sections in all the channels and water level stages under study. Nonetheless, the highest wash load was mainly recorded outside the hydrophytic mats during mean waters, while it was registered within the mats during high waters. Grain size of bed material below the hydrophytic mats as well as flow velocities inside them indicated sedimentation promoted by the hydrophytic mats. Understanding how flow and sediment deposition can be altered by hydrophytic mats is of great relevance not only in terms of the role they play in the Paraná fluvial dynamics, but also for their appropriate inclusion in hydraulic field measurements and hydrodynamic models widely used to predict such dynamics.