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 M ay 1993 in different wetland habitats characteristic of the Rhone Delt a, 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 marshe s influenced by the Mediterranean. Short-term sedimentation was measur ed as sediment accumulation on paper filters which had been placed on the soil surface for several weeks. Total sedimentation and material l ost on ignition was significantly related to individual sampling perio ds, reflecting the importance of short-term processes. High sedimentat ion at riverine sites (up to 22 g m(-2) d(-1)) was related to a combin ation of river stage and wind events. Marine and impounded wetlands of the Rhone Delta experienced low sedimentation throughout the period o f 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, imp ounded, and riverine sites, respectively. Percent material lost on ign ition was low in all habitats (average 15%) and followed a seasonal tr end with a minimum in late fall and winter (1%). Soil percent organic matter was also low in the top several centimeters (13%), suggesting t hat inorganic sedimentation is very important for accretion on these w etland surfaces. Coastal flooding was not a significant mechanism for sedimentation in the marine sites during the period of study. Sediment ation 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 i f management practices continue to isolate these wetlands from riverin e sources of sediment.