Effect of weirs on sediment deposition in Louisiana coastal marshes

Abstract

Sediment deposition both inside and outside of fixed-crest weirs was measured for fresh/intermediate, brackish, and saline marsh areas in coastal Louisiana, USA. Sediment traps, collected on a weekly basis, were used to monitor short-term changes in sediment deposition. Feldspar marker horizons were used to measure cumulative marsh accretion during the 16-week monitoring period. Results show that for most sites less sediment is deposited in marsh behind weirs than at the control sites outside the weirs. Patterns at each site are consistent throughout the 16 monitoring periods. At only one site was no significant difference found. Streamside areas both inside and outside the weirs were found to have higher rates of sediment deposition than backmarsh areas. At both marsh locations, sediment deposition rates were higher outside the weirs than inside. More sediment was deposited in saline marshes than in brackish or fresh/intermediate areas. The vertical accretion data shows that marshes in most areas are accreting sufficiently rapidly to keep pace with local rates of sea-level rise, except for two areas, both of which are inside weirs.