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Substrate characteristics and vertical mat movement were determined for maidencane (Panicum hemitomon), bulltongue(Sagittaria falcata) and wire-grass (Spartina patens) dominated marshes located progressively closer to the Gulf of Mexico in Barataria Basin, Louisiana, at Lake Boeuf, Lake Salvador and Bayou des Rigolettes, respectively. The near-surface substrate of the marshes at Lake Boeuf and Lake Salvador was characterized by high mean organic matter content (> 90%, >80%, gig dry weight x 100) and low mean mineral densities (0.007, 0.013 glee) respectively. Mean dry bulk density at these two marshes was low (0.065, 0.068 glee, respectively). Mean dry bulk densities were highest at Bayou des Rigolettes (0.14 glee); the shallow substrate contained approximately equal densities of mineral and organic material. Rubbed and unrubbed fiber contents in the upper 40 to 50 centimeters of mat were highest at Lake Boeuf (83%,73%), intermediate at Lake Salvador (68%,38%) and lowest at Bayou des Rigolettes (50%,29%). Buoyancy among the three sites roughly paralleled the gradient of increasingly fibric underground biomass. The Lake Boeuf marsh surface was above the water table and responded freely to changes in ambient water levels; the Lake Salvador marsh, while still responding freely, floated below the water surface. The wire-grass dominated marsh adjusted to increasing ambient water levels only to a small degree (3 - 4 em). Water levels and marsh-flooding events were compared among the three sites. Absolute water levels were high during the study interval. Under these conditiqns a trend of increasing duration of continuous flooding and total flooding was evident in an inland direction. The depth of flooding was greatest at the Lake Salvador marsh (measured relative to lowest marsh mat levels). The results presented in this study support the hypothesis of buoyant detachment of attached marsh from the subsiding solid substrate (O'Neil, 1949) as a response to marsh flooding and an absence of mineral sediment. This suggests that floating marshes are an integral and possibly large part of the Louisiana wetlands.