Field observations of the influence of invasive submerged aquatic vegetation on currents and sediment transport in a tidal, freshwater system
Abstract
Submerged aquatic vegetation (SAV) is known to attenuate currents and trap sediment, but there are few measurements of these processes in the environment. The ongoing proliferation of invasive SAV, predominately Brazilian waterweed (Egeria densa), in the tidal, freshwater Sacramento-San Joaquin Delta raises concern that sediment trapping may be impacting marsh accretion and reducing turbidity, which would decrease habitat quality for native fishes. We measured biomass density, current attenuation, and reduction of suspended-sediment concentration (SSC) within E. densa patches at two sites in the Delta. These quantities are needed to model large-scale impacts of SAV, either analytically or numerically. The two sites differed in hydrodynamic energy: Lindsey Slough is a muddy, low-energy backwater, whereas the lower Mokelumne River has stronger currents and sandy channel-bed sediments. At both sites, and throughout the Delta, E. densa occupies regions with water depth less than 2 m, frequently along channel margins. At both sites, Currents were strongly attenuated (85% to 90%) by the SAV at both sites. Low current speeds within the canopy (< 5 cm/s), along with the low settling velocity of the suspended sediment (mostly flocculated mud), limited sediment trapping in the vegetation to less than 1% of the suspended-sediment flux (SSF) in the channel. As a result, the fringing E. densa does not substantially influence SSC or SSF in the unobstructed channel. At the same time, SSC within patches is depleted by deposition and was on average 25% to 30% lower than in the channel. The percent reduction varied with SSC in the channel, from near zero at low SSC to 50% at 50 mg/L in the Mokelumne River. Tidal sediment delivery to marshes is reduced by the same percentage, because the water reaching marshes must pass through the fringing vegetation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMEP057..03L
- Keywords:
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- 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCES;
- 0481 Restoration;
- BIOGEOSCIENCES;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 4327 Resilience;
- NATURAL HAZARDS