Short-term sedimentation dynamics in the Rhone River Delta, France: The importance of riverine pulsing

Short-term sedimentation patterns were evaluated from August 1992 to May 1993 in different wetland habitats characteristic of the Rhone Delta, including impounded and seasonally-dry saline Arthrocnemum marshes, brackish Juncus, Phraginites, and Scirpus riverine wetlands directly connected to the Rhone River, and Arthrocnemum-dominated marine marshes influenced by the Mediterranean. Short-term sedimentation was measured as sediment accumulation on paper filters which had been placed on the soil surface for several weeks. Total sedimentation and material lost on ignition was significantly related to individual sampling periods, reflecting the importance of short-term processes. High sedimentation at riverine sites (up to 22 g m(-2) d(-1)) was related to a combination of river stage and wind events. Marine and impounded wetlands of the Rhone Delta experienced low sedimentation throughout the period of study. Sedimen tation rates averaged over the study period were 0.8 g m(-2) d(-1), 1.8 g m(-2) d(-1), and 5.4g m(-2) d(-1) for marine, impounded, and riverine sites, respectively. Percent material lost on ignition was low in all habitats (average 15%) and followed a seasonal trend with a minimum in late fall and winter (1%). Soil percent organic matter was also low in the top several centimeters (13%), suggesting that inorganic sedimentation is very important for accretion on these wetland surfaces. Coastal flooding was not a significant mechanism for sedimentation in the marine sites during the period of study. Sedimentation is an important factor in elevation change, and this study shows that impounded habitats, the most common "natural environment" left in the delta, may become vulnerable to sea-level rise in the future if management practices continue to isolate these wetlands from riverine sources of sediment.